Pre-liquid cooling, This rig was $1800 (though it could only use 2 of the GPUs so if I had only bought 2 it would be $1300). With the liquid cooling, it comes to about $2600. Right now (given current exchange rate and difficulty) it will generate about $40/day, and I've already generated $650 worth of BTC as valued at the current exchangge rate (which pays for more than two of the cards after deducting for electricity).
The difficutly will increase, but the exchange rate tends to go up with it, canceling this effect. After about 50 more days I'll be profitable (though if you count the value of the hardware, my profits have already outpaced depreciation, so I'm profitable in that sense).
If I had gone with just two cards, and kept it aircooled and running during my down-time, it would probably have already paid for itself.
Do you know why the ATI boards perform better? Nvidia has devoted more transistors and R&D to their GPGPU functions. Has no one coded directly to CUDA?
There seems to be quite a bit of a Bitcoin interest around here, with several articles about it already: [1 2 3 4 5 6 7]
I propose that links and generic Bitcoin comments should be posted here, instead of making a new discussion thread for each interesting article about the subject.