With my x2 3800, 7600gt, 2Gb of RAM I can hit max settings at a 1152x864 resolution. I get anywhere from 15 to 30 FPS most of the time it hovers in the low to mid twenties. If you want any more than that you''ll need a better video card.
Im running it at full graphics at 1028 x 768 25-30 fps on dual 2Ghz CPU with 6800GT 256mb oced to 435Mhz so any series 7 card should manage it. Some of the graphics run on the second CPU core.