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I got access to a huge server farm where we do our server hosting, we actaully got a few empty full racks and i received confirmation to start this venture.
we don't pay for electricity and it is included in our monthly pay. the place is air cooled with extreme chillers and you actually need to wear a coat when getting in or your nose start watering. its freezing in there.

anyway, I want to start doing mining and take advantage of this free bandwidth and electricity.

1. What hardware should I buy that would be the most cost effective and return the investment in the shortest time possible?
2. since there is no heat problem and the noise wont bother anyone, I guess i can put multiple cards in the same PC.

thanks for your help and good luck everyone.

AnalogKiD

If electricity will never become a factor then get as many 7970's 0r 7990's when they come out as you can afford.  Although if you use a lot of electricity your rent may go up so maybe you want to look into BFL's or the Sparten6 FPGA boards which cost more but use even less electricity.

Electricity is included in the rent.
We just signed the contract 4 month ago and it stay valid until DEC 2013.. maybe then they will start charging us for the KWs

With that kind of access - just buy as much hashing power as possible!
That has the potential to make a BIG killing...

Do you know how much (and what kind of) power is delivered to each rack?  120V?  208V?  48VDC?  20A?  Multiple 20A circuits?  Multiple 30A circuits?

As someone who's run data centers before, I can tell you that even with "unlimited" power, you run into issues of power density - how much power can be delivered to each rack and cooled effectively.  This almost certainly will affect your planning, so it's worth looking into.

well..

we get 2 X 20A on each rack (220V) and we got about three empty out of 10.

I just ordered a 4 x 7970 rig to begin with.. we'll check the ROI on that and we'll decide on future purchase.
If we'll see the light we'll probably add a fifth one and maybe buy a couple more rigs but that depends on the cash flow coming from the first rig.
the thing is the the initial investment is pretty high.. around 2.8K for each rig (4 x 7970 = $2000 + $800 PC) so we are still concerned we won't be able to return the investment fast enough to purchase more.

do you suggest solo mining or pool mining with such power?
what do you think the BTC/Month rate would be (apporx.) in the first few months?

Thanks for your help.

Since you dont care about performance/W  you need to look at performance/$ and then 5830/5850/5870/5970 is your best bet. That means buying second hand, but it will be considerably cheaper than 7970s.

As for how many BTCs it will earn. Rather shocked you didnt do the math before ordering anything. Its not rocket science:
http://www.alloscomp.com/bitcoin/calculator.php

Of course the big question is what difficulty will do in the future. I spot a trend:
http://bitcoin.sipa.be/speed-lin.png

I Did check that.. just wanted to get some confirmation.
what about solo vs. pool ?

what do you thing is right for 3Gh/s ?

Rather obvious: a pool. On average you would find a block only ~once per month. With some bad luck it could be 10 months before you find one.