definitely NOT folding;
so that leaves it running idle and using as little power as possible :)

If I'm going to pay for the GPU to sit idle, then I might as well pay the ~$5 difference to have it doing something useful per month.

I'd rather use that extra $60 to buy a better GPU ;)

Will a MB be able to support three of these?

don't see why not, as long as the PCI Express slots are far enough apart

I was thinking more on the lines of transfer speed. Can the CPU keep up?
Somewhere I read that more than two GPU cards ( 9800s and better ) was a waste.

that's because GPU scaling decreases very quickly after 2xGPUs; CPU bottleneck is not quite the issue

Multi-GPU scaling = diminishing returns... 2nd isn't too bad with mainstream games, but 3rd rarely helps significantly and 4th is there mostly for looks.

I'll probably build the system with four low end cards and get some test results before throwing money at four fermi cards.

If that overloads the system, I'll move two to another MB.

What do you expect to gain from four cards? Unless you are driving two 30" monitors anything more than 2 GPUs is going completely to waste.

Keep in mind the majority of games will not scale beyond 2 GPUs because the software wasn't written for it, or they aren't stressful enough. A simple GTX 260 is enough to drive a 1920x1200 with all settings maxed in pretty much any game. And unless the game is specifically coded for SLI by NVIDIA, then it will only use a single GPU.