The game is a CPU hog, simple as. It doesnt really need much in GPU power (just vRAM). I had a X2 6000+ once, and GTA4 was playable on it, with drops when things got heavy. I then got a Q6600, and it gave me a nice little boost on FPS, and the frame drops disapeared (more threads or whatever for the game to work with). The only CPU's ive seen running the game at 60 FPS are the i7's. Also, ATI's drivers or Rockstars coding for ATI is epic fail, the game runs better on lesser Nvidia hardware than some of ATI's top stuff (not that its related to OP as he has an Nvidia card, just saying

).
I agree the game came out in a dire state, but its been getting better since and Rockstar do seem to be trying to fix up all the problems people have been reporting. The game seems very stable and runs well atm with the 1.0.3.0 patch.
Edit - Also just to add, ive been playing the 360 version recently, and i was surprised how much the FPS dropped here and there, worse than my X2 ever did, meh, the game just loves CPU's

.