Like I said in the last thread, I'm looking into putting together the same type of "folding farm" as in that picture above. The idea in this link looks like a good option for the power. Make ATX Y Cable by splicing two cables together so you can run two MB off a single Power Supply.
http://forums.extremeoverclocking.com/showthread.php?s=&threadid=33884
How much power/money is used by keeping a F@H running 24/7 on a computer? Roughly, a CPU uses about as much power as a watt light bulb. Here's a report on computer power management from Lawrence Berkeley government labs, and there are other references on the web you can find. Although power supplies on most computers are rated at 400 watts, average usage is lower.
On average, a Pentium-type computer uses about 100 watts (if the monitor is off). So, the daily difference between off and running F@H is about 24x100 = 2.4 kWh. At $0.15 per kWh ( from PG&E here in California), this works out to about $0.36 per day. In general, lighting and climate control use a much larger share of household power than computers do. So the best bet for cutting costs and conserving energy would be to turn off lights, turn off your computer monitors (which use more power than a CPU), and turn down the heat.