Be warned, GTA IV still runs like a dog on my Q9650 with a 4890 that has 2 gigs of VRAM. I'm sorry but the game just does not perform properly on PC, considering the graphics. There are far more good looking games that perform far better on given hardware.
The game is massively CPU-limited, they must have the CPU doing all kinds of things that could be done by gpu. Also, they limit the options based on VRAM, yet even with 2 gigs of VRAM here, it performs poorly when outdoors in-game. And this is only at 1680 x 1050. Forget about getting 60 FPS, not with this game, it just doesn't work like that.
It's a shame because there's some great content in the game but you really need a quad-core CPU due to the way the game was ported from console. A GPU upgrade would be better for gaming in general but, alas, not for this game.