Eurogamer has published an early review of GTA IV for the PC and complains the game is pretty hard on the hardware. The reviewer only had a 2.4GHz dual-core system with a GeForce 8800 GTS 640MB and 2GB of RAM, but what he writes seems to suggest GTA IV's performance on the PC may not be that good as most of us were hoping for:
But on other levels, it hasn't a clue. Like the vast majority of PC games, it auto-calculates its graphic settings. Unlike the vast majority of PC games, which generally underestimate what your PC can manage for the logical reason that a less-attractive game is less annoying than a less-playable one, its auto-calculation is more than a little optimistic. I had to knock down the default graphic options quite a bit on my PC (dual-core 2.4GHz, 2GB RAM, 640MB GeForce 8800) until I got an acceptable rate, and even then there was the occasional really nasty drop. While my experience was more pronounced than most, speaking to other reviewers via the secret review-grapevine lead to confirmation that fiddling to get it playable was universal.
That said, it's worth noting that even turned down, Liberty City remains one of the most visually stunning places yet seen in videogames. And, in terms of future-proofing, there's a lot of sliders that can be turned wayyyy up. In five years' time, the PC version will be the definitive visual incarnation of Liberty City - playing with a draw-distance set around to 20 and traffic-density way low makes the prospect of those hitting the 100 exciting. But now... well, it probably still is the most attractive version, but it's annoyingly fiddly to get there.