I can appreciate what you are trying to accomplish, but I cannot follow the logic.  If the guy is a scammer and does not deliver why would there be any outstanding debt to him at all?  Why would those scammed later and likely to lose their money take a further loss and pay to those scammed earlier?  You are effectively selling his debt to you but how do those buying the debt collect from him if you cannot?  If he is doing any legitimate work for anyone, do you think it's going to fly that they say "I just paid hundreds of dollars for your work to some people on the bitcoin forum that say you owe them . . . now GET BACK TO WORK."  On the other hand, if people owe him and want nothing further from him AND feel morally compelled to pay someone, I suppose it should be you.  I now NOTHING of this situation, but I wouldn't hold your breath waiting for strangers to pay his debt to you.

I have read that Bitcoins produced will be 30% less in the next 3-5 days, as the difficulty level will rise again.  True?  All the people dumping money into 5870's and 5970's very recently are going to need that much more money per Bitcoin to recover their investment on hundreds of dollars (if not thousands in some cases).  Also, with all the new entry, sales of "hash contracts", etc. does it not become that much harder to compete for the Bitcoins since only so many are produced over a given period?  Marginally, when one compares cost of electricity to current sales price of Bitcoins it seems like a good deal.  But adding the high equipment cost into the equation, along with increased difficulty in mining and HUGE recent entry into the market, I predict Bitcoins are stable at a US Dollar and will quickly go up in a very short time.  What do you think?

So long as BitPenny is just immediately selling on MtGox at market rates, I fail to see the conflict.  As I cannot define the market bottoms and what is considered volatile in the market, for either myself or BitPenny to refrain from immediate sale upon generation would be currency speculation, which does not enter into the business model proposed.  However, the poster who said just pay PayPal for hashs hit the nail on the head.  THAT I would be willing to pay 10% for to eliminate loss of time, and possibly Bitcoins, with all the given transfers.  For example, right now I want to transfer $100 out of MtGox to LibertyReserve and cannot do so because I believe the owner of MtGox does not currently have funds on deposit at LibertyReserve, so I have to wait.

Also, PSU's jump in price as you get towards the highest wattages.  And you need more and better cooling in your box.  You may be able to pick up an old ASUS A8N32-sli (what I use) w/ cpu and memory for $200 or so on ebay.  Weigh the few hundred extra against the time and trouble spent trying to do it all in one box.  At current prices, you'll make it back in the week lost trying to get it working in a single box.  Let us know if you get it working though.  Very interested.

If I had to toss a coin, I'd say that a 1200 Watt Corsair or Antec might do it.  Might.  However, it appears you are using the MSI 890FXA-GD70 motherboard.  Though it has five PCI-E, they appear stacked right next to eachother.  I tried TWO right next to eachother in my MSI Big Bang, which has as many or more slots, and found that putting cards in two adjacent slots caused the heat to go WAY up, perhaps 10-15 degrees on each card, causing malfuncitions.  The clearance between the cards is minimal, so I have no idea whether aftermarket cooling is an option.  I had to space the cards out enough that I don't think I could get a third one in there without more spacing between the slots.  If you get 4 cards in one box working reliably, please say what the trick is.  I noticed about 91 degrees is about where my 315khash begins to fall apart on each core.  Also, I did find that things run well at 99% GPU, but when I get to 100% GPU it falls to about 200khash/second momentarily until it drops to 99% again.  This is due to the increased heat just a couple of degrees above 91 C.  If I turn the clock DOWN, the GPU will cool, stay at 98-99% and give me a consistent 315khash again.  If you get higher numbers, let me know your settings.

I'm using the Corsair 1000 that I posted a link to and an Antec 850 in two different machines running Win7 x64 in each with 2 5970's.  Both good power supplies after trying about five of them total.  Keep an eye out for the fact that some lines, such as Thermaltake  I think, actually have different lines of PSU that they release under the same wattages.  That alone should be an indicator that an 850 Watt PSU may not put out 850 Watts, even when comparing to the same brand.  Check online and find comparisons and reviews especially if it's something you are buying online and can't try out and easily return to the store.  I am an extreme cheapo when it comes to components but this is one area where you cannot just compare Watts and price.  Trust me.

I literally want a bell or whistle to go off when I found a block.  Or I want the Bitcoin program to play an mp3 of "I'm in the money".  Or maybe just a loud siren accompanied by the screen flashing.  Or maybe the computer can catch fire.

I don't know what kind of GPU you are using or what kind of CPU you are using but I have found that I get LESS hashs if I turn on 'Generate Coins'.  This is likely because the amount of CPU power taken away to generate coins negatively impacts the CPU's handling of what the GPU is doing.

And just FYI for people coming to this thread, bear in mind that PSU rating vary wildly.  Certain 1000Watt PSUs will NOT power 2 5970's, while other 750's certainly will.  I have personal recent experience with this.  If you want the short advice, read whether the PSU is peak rated or constant rated and even then it's a crap shoot.  Better yet, just stick with Corsair.  WalMart has a 1000 Watt Corsair for $219.82 delivered to your local WalMart.  http://www.walmart.com/ip/Corsair-HX-Series-80-Plus-1000-Watt-Certified-Power-Supply-CMPSU-1000HX/12457257  Can't beat it on price or performance for this particular task.

I had something similar with an older ASUS A8N32-SLI where it would not recognize 2 5970's.  Then I realized that CCC was only showing one enabled that I had plugged the monitor into.  Thus, I would plug the monitor into one 5970 and start the program for device=0 and device=1 and that 5970 would work.  Then I'd unplug the monitor from the first 5970 and plug it into the second one and which point I'd rerun the program for device=0 and device=1 again.  That's the only way I could get it working in four DOS windows on Win7 x64.  My newer MSI Big Bang mobo runs all four in SLI mode with the SLI cable connected on Win7 x64.

You may try plugging your 3rd card in, get the first two working then plug the monitor into the third (check in CCC to see if it went active) then try running the program on the third board.  Could work.

Incidentally, I think you are in the UK, so I have no idea what you pay for 5970's (if that's what you use) but Frys here sells them for $599 with a $50 ongoing rebate from the manufacturer (Diamond), so $549 each.  Hard to beat, even online.

Also, re-reading your post, you are speaking of economies of scale.  It costs a certain amount for me to run a couple of GPU miners.  It would cost less, per mining unit, for me to run five of them.  After that, I suppose I'd have to run new wires in my walls to carry the increased amperage and eventually have to move to  a data center, which would drive the price way up.  So, yes, your actual costs to "do it right" large scale would be much higher than I'm quoting as well.

I do agree that my analysis does not take time and opportunity cost into account.  You are obviously an educated computer programmer with a diverse background and your time has a value and a cost.  As a full time job you might be able to make more doing other things rather than mining.  I was strictly figuring hardware costs and electricity.  I myself have 4 5970's running 24/7.  If I factored in all of the time spent going to and from stores to get the components, building the systems, getting the software running and tweaking the programs ... and added that to my cost, I would have to conclude the venture was not worth it at all.  

Having said that, I do think that so long as the upfront cost of the equipment is more than paid for by those buying contracts for GPU time, there is still a nice profit to be made by someone who values their time and skill set less than you do.  And where someone sees a profit, someone will act if there are no barriers to entry to compete.  Upfront cost is a barrier to entry in most businesses that does not appear to exist to a large degree in this case.  If someone would give me $2000 I could build them a box that runs two 5970's and I'd make a very small profit.  Better yet, I buy the box, plug it in at a little over $100 a month and break even in less than three months given current prices and difficulty.  Better still, THEY buy the box and I give them what I generate over the first three months.  Depends how one values their time, I guess.

To answer the question in the topic . . . Four.  Four Bitcoins is enough.  The rest is just for show.

Before responding, just so you know where I'm coming from, let me say that you are well respected and known around here and I think there is very low risk in your offer and it is a genuinely good one that I would myself invest in if I had the money.  Your offer is a 'win-win' for both you and your customers.  But . . . competing with you would be very easy, I think, because the upfront investment is made by those buying the contract and, as pointed out in past posts, the contract cost well exceeds the equipment cost.  In fact, if I were as well known and trusted as you I would compete with you myself (if my mom would not mind losing the basement space).   But people don't know that I'd be around in three months, much less a year, or that I'd evaporate into thin air with their money. [EDIT- I meant "would not evaporate" ... Freudian slip?]  That is your advantage at this point.

And, as I previously said, I hate to even mention it because if my statements encourage others to compete with you, they are competing with me as well, since everyone mining is in competition with eachother.  But my gut still tells me that competition is a good thing for both of us on some level.

I currently generate alot of Bitcoins, so 10% would not be worth it to me.  Your commission would add up to several hundred dollars over just a few months, instead of the pennies paid by CPU miners.  However, if you added this to a service which could immediately list and sell the coins on MtGox for market price (which costs just over half of one percent) and then exchange the proceeds and have it delivered to PayPal (which PayPal does for free, I think) then this adds only the Liberty Dollar (MtGox uses them) fee of 1% plus whatever fee you could negotiate with a currency exhanger (a couple of percent?)  

Thus, you'd still make 6-7% (still hundreds of dollars from me to you) but you may get some of the big Bitcoin generators like me who just want to calculate hashes and get cash (sorry for being a capitalist folks).  I would personally jump at this as it would remove from the equation having to:

1) Wait for blocks to mature to get payout from mining pool.
2) Log into MtGox and enter the amount I want to send MtGox.
3) Enter the address MtGox gives me into my bitcoin wallet and send to MtGox.
4) Wait 15 minutes for those funds to make it to MtGox and then place a trade order.
5) Take the proceeds, less .065%, and forward those to LibertyReserve (which MtGox requires).
6) Use the lengthy LibertyReserve login process to see funds have made it, less 1% for the liberty dollar I never wanted.
7) Sell the liberty dollar through an exchanger for a few percent.
Get money in PayPal - PROFIT!

Skipping directly to step 8 would be nice for me.  Maybe you can cut a deal with MtGox and eliminate Liberty Dollar entirely, saving 1%.  Cut a volume deal directly with someone who exchanges Bitcoins for PayPal delivery.  This would be an incentive for MtGox as well as you would drive business there exclusively.  Cut out all of the middlemen.

In fact, if the increase in difficulty in mining could be calculated with reasonable probability, you could just sell futures contracts outright.  $3000 today buys you 5000 Bitcoins in March, for example.  You would have many more takers.  A moment ago I wanted your house to burn and now I 'm trying to help you get business.  Go figure.

Saw this thread in your sig and checked it out.  Interesting business.  At first, I thought, why is he betting against the Bitcoin?  This because, as pointed out, at current rates it is a good deal contrary to prior posts.  People are basically buying a futures contract on the Bitcoin.  But why, over the term of the contract, would you give up Bitcoins that are worth more than the value of the contract?  Simple answer . . . without the contract you would not have the future Bitcoins since you are using the contract money to buy the equipment.  At the end of the contract, you don't have the Bitcoins generated by the 5970's but you DO have the worlds largest private collection of 5970's.   Next to China and the US, you will probably have the worlds next fastest super computer.  Good business model and no risk. [I have a similar business model myself - I call it my credit card.] Hope someone competes.  Then again, I hope they don't and I hope your house burns down in an electrical fire, since you make it easy for non-technical people to mine Bitcoins and you are in competition with me :-)

Here's what I've observed with a 5970 and now a 5870 as well.  Hope it helps someone.  First and foremost, a few posts back someone noted to use the "Known Best Settings".  I added the -f5 switch and I went from about 560Mhash/sec to consistently over 600Mhash/sec on the 5970 alone. 

Next, the first post in this thread says . . .
"5970 is actually two 5870s with internal crossfire. OpenCL doesn't work with crossfire. It must be switched off. You can't switch it off on Windows. For now, 5970 can be used fully on Linux only."
I have used the 5970 for over a month on Windows 7 and get results from both cores, about 300Mhash/sec each.  So I don't know what that comment is all about.

Next, I finally was happy with my results with the 5970 and I can never leave well enough alone.  So I bought a 5870 tonight and threw that into my motherboard as well.  At first I was getting "Disabled Adapter" in Catalyst Control Center, although I could see the third GPU (now as GPU #1) in MSI Afterburner.  It did nothing and just sat there very cool.  poblcm only recognized the two 5970 cores and the CPU.  After looking at posts on Google as far back as 2008, someone said, "Plug in another monitor".  Not having another monitor, I plugged in my monitor to the 5870 instead.  Now, the 5870 GPU was the only core recognized by poblcm along with the CPU.  So I ran poblcm with a device=1 and it worked!  Then I unplugged the monitor from the 5870 and replugged it into the 5970.  Now, even though the 5870 was the disabled adapter again, poblcm was still running on the 5870.  Then I ran poblcm again on device=1 and device=0 and both of the 5970 cores were running alongside the 5870, for a total of about 875Mhash/sec.  Pretty good.  So that's just FYI for anyone running multiple 5870's, 5970's or combinations thereof.  I wish there was a way to get Catalyst Control Center, or ATI Stream, or whatever ATI driver it is to enumerate all three cores so I would not have to unplug and replug the monitor into different cards every time I want to run poblcm on the 5870 and 5970 at the same time.  I would suspect this would be a similar problem, and a similar solution, for running poblcm on  two 5970's on Windows.

Most of you must be new.  The fact is that there IS no Satoshi Nakamoto.   Bitcoin was actually started by renegade anarchists for the Danish company Aller Media, specifically those working for its Finnish publication "Oho!"  They envisioned a currency that would run amok in the world, without central control.  Some of them are reported to be Satan worshippers.  In fact, Satoshi Nakamoto is simply an anagram for Satanist Amok Oho!