As has already been mentioned numerous times, GTA 4 is much more CPU intensive than any other game before it:
GTA 4 with 13 CPUs compared
GTA 4 Core i7 comparisons
What's funny is how people complain there aren't any games that make good use of multi-cores or multi-threads, then once one comes out, everyone who doesn't have the necessary hardware bitches and moans because the game runs like shit on their PC. Minimum and Rec'd specs have typically still understated actual hardware requirements, so the people bitching that don't even meet the minimum really need to stop being the weakest link and upgrade their hardware or gtfo.
Its always going to be that group of hardware minimalists holding back the development of games as devs need to balance performance and hardware. In the case of console to PC ports, the developers are clearly leveraging the strength of consoles to maximize visuals and performance by making full use of their highly parallel RISC CPUs and make up for their disadvantages elsewhere (GPU, cache, system RAM). I'm not complaining of course, as many PC titles going forward are going to be console ports that will look and run better on the PC with the necessary hardware.
People trying to compare the PC to consoles and complaining, you really can't, as the PC version is superior in every way, even on sub-par hardware. If you want to make a more accurate comparison, run at 1280x720 (max resolution supported by the 360), turn view distance, detail distance, car density and shadow density to the absolute minimum settings. You can even leave texture detail, render quality, shadow quality and water quality higher if you have a decent GPU (DX10 or better) as the game isn't GPU limited before its CPU limited. Guaranteed you will get higher than 30FPS on a decent dual core CPU and it will still look and run much better than any of the console versions.
GTA 4 Console vs. PC Image Quality Comparison
Also, FSAA does not work for those wondering, maybe if there's ever a DX10 version released. The game uses deferred rendering and shading along with heavy alpha to coverage sampling for transparencies. The Smooth Shadows tweak I linked earlier actually looks like it increases the level of Alpha Blending, which essentially smooths edges and decreases aliasing on alpha textures, which isn't bad given traditional MSAA and TrAA methods don't work.
Smooth Shadows Off
Smooth Shadows On
Notice the big difference in the trees, fences and shadows. There's also some light edge smoothing throughout the scene.