Just to add another configuration to the list for reference:
Q6600 at 3.5GHz
8GB DDR2 1066 (running about 1050 ish currently)
2 x 4870 1Gb crossfire
30-40 fps at 1920x1200
Previously set ups:
Q6600 at 3.2Hz
4GB DDR2 800 (GTA4 used so much that there was a delay when going back to desktop - gone with 8GB)
1 x 4870 1Gb or 2 x 4870 512Mb
30-35 fps at 1600x1200
I always run the settings as high as the memory limiter will let me (changing View Distance to get just under the limit), so everything is on High or Very High.
With most games, so long as your CPU is high enough not to bottleneck, your GPU determines your FPS. With GTA4, it's the other way round - so long as your GPU isn't TOO slow, it's your CPU that determines your FPS. This is why some reviews found that even changing resolution to ridiculously high didn't make much difference to game speed.
I've never tried gaming with an i7, but from my little explorations around changing memory and graphics cards I firmly agree with most of the other posters - you'll need a fast i7, 4-8Gb of DDR2/3 and a HD4850 or above to even try to hit 60fps. Don't know whether it'll stay there when you're actually playing though.
GTA4 really is a beast of a game on requirements, but when it does play well - WOW is it worth it!!!