why do you want 100 frames a second? there is no point, your screen will only display 60 frames a second anyway. and you would probs only need a 320mb 8800gts, not a 640 to get that. also you wouldn't need a CPU as good as dashogun is suggesting, that would be way overkill imo. you could probably get away with a really crap CPU that isn't even dual core! my mate plays his CSS on full settings with like a 7600GT i think... with a 3ghz P4, but that is @ 1280x1024, which is probably around the same pixels anyway. seriously, you dont need an amazing computer to run CSS really well.
If you are getting an 8800gts then a matching cpu would probably be around e6400 or something. there's my 2 cents.