from my own experience, it seems the CPU is the limiting factor when you go above 1 wow per core (with a e6700 @ 3.33 ghz)... 4 instances on a 1920x1200 screen got me about 30FPS if i recall under Vista 64 with a 8800 gts... pretty lousy.

really I'm only happy with refresh rate FPS so I was dissapointed when I bought a 280 GTX and couldn't even get that for 2 instances, again under Vista64 (main on 1920x1200, 1x alt on 1280x1024). Vista is just shit for FPS, that's my conclusion since even my 7800 GTX got better FPS under XP!

i dunno about those 8600's, but i'd get a quad core!