I just think you'd get the best bang for the buck if you used the money for (2) dual GPU cards per machine...and built 2 of them. If you really want to push it, try three of them in a rig! I know this can work with Linux, I'm not sure about Windows - I keep getting conflicting reports on that (running 3 dual GPU cards for 6 total GPUs).

Honestly, for max credits all you need is a (mid grade) CPU that can push the GPUs enough - like I said, an X6 AMD 100T for $199 or an i7 (around a 920 or so) for a little more.
A good mobo with 3 PCI-e slots and lots of bandwidth is only like $150ish...
Don't spend more than $60 on the HDD, 4GB RAM for about the same...could be really inexpensive except for the high end PSU and the GPUs.
And even then, you can get a solid 1KW PSU on Newegg in the $100 - $150 range easy.
I'm all for the crazy stuff - but I just hate to see a guy spend tons of $ when he doesn't have to...