So some of that would benefit from CUDA, and depends on your budget and level of performance.
People though are saying now is not the time to invest in building a PC when Intel seems near to having new processors and support for Ivy.
You are limited by our PCIe 1.1 bus though and that will always hurt, limits even buying flashed PC cards.
Ideal of course is even pre-built PC and throw in a good card.
Something with 2GB or more VRAM. CS6 users for example want GTX 570 w/ 2.5GB (or more), and 580 are better for now.
Someone did a write-up on how they got SLI working with theirs. But they have PCIe 2.x and two 16x slots.
http://forums.macrumors.com/forumdisplay.php?f=1
I assume you got past the issue with installing and booting Windows 7 or later DVD (I cheat and use a PC then move the Windows drive before it has restarted over to my 1,1).