You'd probably be good with 2 gigs of ram, 3ghz processor and a 256 mb upper end video card.

I don't see how if mine can run fairly well on large unit settings with everything turned up (I've even turned grass all the way up) and I have a 2.7ghz celeron with 768 ram and 128 mb lower end video card, that you would need much more to just go to huge settings. I may have actually said too much.