Toms Hardware (referenced earlier) is a good site. Here's the October GPU hierarchy listing
http://www.tomshardware.com/reviews/...ce,3042-7.html
I'm running the EVGA GTX570 suprclocked with 1.28Gb memory (nearly a year old now). It runs well at High settings, 40-60fps (I turn Grass off) but the more memory you can afford on the GPU the better.
I was looking at the new 6xxx ATI series back then but I went in favour of Nvidia as I've been with them for some years and never a problem. I believe there are some issues with the ATIs (SLi??) but others can tell you what or you can search the forums.