I just bought a 5850 for it, and using it along with athlon 635 @ 2.9ghz. Its fine under 1920x1080, but you wont be able to max out the settings. I know my cpu is bottlenecking, but it seems to me 5970 is needed if you wish to max it out.
I also heard that this game favors ati over nvidia. I will try later with my geforce card to see how big a difference there is.