I never said you cannot use crossfire, I merely said it won't increase your hash rate at all.
Moreover, spreading the GPUs further apart (if possible with your mobo) might drop the temperatures by a couple degrees.

GUIminer provides merely a graphical interface, which mining kernel do you use? Poclbm? Phoenix?

SDK 2.5 is the optimal version for VLIW-4 cards. The lastest drivers come with SDK 2.6.

You already installed both of them by installing GPG4Win.
The default installation directory for Gpg4Win is C:\Program Files (x86)\GNU\GnuPG  (on 64-bit systems) or C:\Program Files\GNU\GnuPG  (32-bit systems)

If you cd to the installation directory both gpg.exe and sha25sum.exe will be in scope:
The last line is no mistake, you can use the backslash to denote the root directory of the currently chosen partition making the obnoxious c: drive letter redundant.

I recommend that you add c:\Program Files (x86)\GNU\GnuPG to your path so that sha256sum is always in scope.

Driver version? SDK version? Operating system? Miner?

There's a whole suite of miners and mining kernels, each with its own range of optimal settings like vectors, work size, aggression/intensity... unless you're more specific about your system configuration, I don't believe anyone can help you.

Crossfire is totally irrelevant for mining. Thermal management will be easier if you are able to remove the crossfire bridge and spread the GPUs farther apart.

Since you need uncomfortably high fan speeds already I discourage you in the strongest possible terms from increasing the powertune parameter.
Fanspeeds over 70% are not only unpleasant but detrimental to the fan MTBF. Running the GPU fan at 100% is a surefire way to destroy it in a couple of months.

Take your time and browse through Mining and its subforums.
The Pictures of your mining rigs topic might give you some good ideas about optimizing the airflow across the cards.

I suggest you sign up with BitMinter
You can very easily launch their custom miner with a mouseclick.

With your current set-up you'd best forget about mining.
By the end of the year you might need to pay the equivalent of that engagement ring in power costs if you proceed with the nVidia cards.

Umm... that's how I do it, only with command line programs:

First of all, it's the SHA256SUMS.asc file that needs to be verified, not the executable itself:

When gpg is happy (gpg: Good signature from "Gavin Andresen (CODE SIGNING KEY) <gavinandresen@gmail.com>") you can take a peek inside the SHA256SUMS.asc file.
You will find, amongst other things, this line:
What it tells you, is that the current Bitcoin installer package for Windows has a SHA256 checksum of 6943830d0cc1e6514297d761017007c23da365c6b4f0e8e769a5a131825e5b32.
The digital signature guarantees this information is correct and has not been tampered with.

Now let's calculate the checksum of the actual installer:
6943830d0cc1e6514297d761017007c23da365c6b4f0e8e769a5a131825e5b32

Done.

HashCalc is a great and free GUI tool you can use to calculate pretty much any checksum you need.

Con, you might be interested in P4man's solution. Fastening the fan with duct tape won't void your warranty.

BTW, whatever did happen to that PSU of yours? Feel like sharing some data points? What make and model was it? What load did it typically pull?
Oftentimes, glitches at the production plant like solder points with inadequate amount of solder won't be severe enough to render the device completely inoperable but they will make themselves known (perhaps even with a bang and some fireworks) as the load increases.

If Windows falls flat on its face and Ubuntu dies in pain, what makes you think that the faildrivers won't take down OS X just as easily?

You need to learn some basic sysadmin tactics: never-ever deploy untested updates on production machines.
Your miners are your production machines, leave good enough alone - find a working driver/sdk combo and do yourself the favor of never touching it without a very good reason.

I've once seen my Debian systems "lose" the AMD driver following a slew of system updates. Fglrx module could no longer be found, manual reinstallation of the driver package helped.

A highly overclocked 6750 (basically the same card as your 5750) can pull 206 MHash/s.
This low-end card can do a better job at mining than two of your nVidia cards put together and at a fraction of the overall power usage.

Why build another PC? Either put the two nVidia cards together or simply sell the 560Ti.

Other avenues for coin generation?
Invest into an FPGA miner - that's the only way to be future-proof today.

Two machines each armed with a single nVidia card is a truly horrible set up, to the point of being laughable as anyone's idea of a mining rig.
You're burning up much more money than you'll ever earn unless the exchange rate skyrockets to $30, which isn't exactly the most likely of scenarios.

Don't mine on slush, the damned pool is not only hoppable but it's been exploited for a very long time.
Find yourself a nice zero fee DGM or PPLNS pool and mine there. I've been very happy with BitMinter, Ozcoin and Eclipse. Those pools are rock solid.

The root cause of the change is that AMD drivers are composed of loosely connected sets of technologies:
ADL - used for clocks, voltage temperature and fan control - being hardware-bound, sees only one device.
OpenCL - the language used for GPGPU ops - sees one device per each connected VDU.
Unfortunately, no one has been able to reliably tie ADL devices to their corresponding OpenCL virtual devices or in some scenarios (some systems using multiple double-GPU cards) even to the correct physical device!

I'm not exactly sure this matter requires any action on your part but if you wand to do something about it, one of the virtual openCL devices can be disabled with disable-gpu.
The number of threads per GPU core can be specified with gpu-threads.
It's all pretty well documented in the HOWTO.

So, the technology saved you today.
Please be careful, you might get less lucky next time.

This data can be mined out of the blockchain but I'm not aware of any project meant specifically to monitor the current situation.
BTW, doesn't the huge number of 1-tx blocks by Eligius⁽¹⁾ seem odd?

Notes:
(1) Supposedly. Blockchain.info has been known to make mistakes before.

You must mean transactions, not tx fees.
I don't believe so, but the issue of not including transactions is far less serious than some give it credit for.
Mining empty blocks will at most slightly increase the time transactions take to get confirmed.

There are two larger risks botnets pose to Bitcoin:
(1) they could be employed to mount an array of disruptive attacks, possibly even the 51% attack
(2) they damage the economy creating downward pressure on the price. Unlike the honest miners, botnet operators can afford to sell their coins at $1.

Four Bitcoin blocks mined today definitely improve the pool's luck, good job everyone.
Can we get a couple more until midnight?

116 000 immobilized zombie machines is definitely good news.

Does it come as any surprise that the bulk of the machine population were running the dated Windows XP?
Neglected, unpatched and unprotected machines are not only a risk to their owners but to the Internet as a whole.
If only the owners were technical enough to realize that...

Did the overall hash rate diminish?

Reading the FAQ doesn't hurt, really.
I take it you have familiarized yourself with the README, right?

GUI Miner is just a graphical interface. Which miner are you using? The actual miners that come with Kiv's Windows package include poclbm, phoenix and rpcminer.

Your stale rate depends on the pool server, your network connection to the server, and the miner configuration.
Pushing the Intensity (Aggression) too high for the card's hash rate will increase the stale rate.

*sings out loud* An Ada client, BASIC code pointerless and cross-compiled, why?⁽¹⁾

Notes:
(1) Line supposed to be sung to the melody of System of a Down/I-E-A-I-A-I-O  

Naturally them bitcoins aren't green - they be golden, silly.
They might get green-ish when they lay around for a while, like the bacon in my fridge

To be sure, there must be a good reason why miners get VIP customer status at their power companies and some of them even lovely thank you letters attached to the boxes with the invoice print-outs, but on the other side the miners are also the only people I heard of who have calculated the energy consumption of their entire households and seriously contemplate the idea of shaving off a few watts per year by removing the status LEDs from all devices.
Even the most miserly of Scots don't get that far - they are entirely satisfied with disabling the fridge light bulb.



Seriously though, we're talking of quite possibly the most innovative project since the damned TCP/IP protocol, man.
A project that could very well integrate the financial system and the Internet very tightly without the need to trust any third party.
Surely, mining looks far less wasteful in this context, doesn't it?