I've owned the PS3 version of the game since it came out, however I wanted to experience GTA IV in all of its 1080p glory so I purchased the PC version from Steam today. I thought my computer would be able to run it at max settings with no problems but I seem to be mistaken as it stutters quite a bit when I am driving fast. My specs are:
Processor: Intel Core i7 X 980 @ 3.33 GHz RAM: 12 GB GDDR5 GPU: ATI Radeon HD 5970 2 GB Resolution: 2560 x 1440 OS: Windows 7 64 bit
If my system can't run it at max settings what exactly does it take!?
More than what you have? But seriously, this game hasn't got the best optimization, and it was designed to be future proof so in a few years time it won't look too outdated.
Looking at your post, you say it stutters when driving fast. Does the game run ok when you're not moving or driving slow-ish? If so, then it could be a hard drive bottleneck as the game can't get the data off the drive fast enough, so it will stutter as it loads things and renders them. Of course I'm only assuming here so I could be completely wrong.
You could also try turning down some settings to see how that affects the game. Some settings take more resources to process than others, so a small change might make a big difference.
When I'm just walking around the city everything seems fine. Although if I turn around really fast I see all the textures load in, but that's a minor annoyance. I don't notice much of a problem driving slowly either. However when I really get going fast it tends to just get really choppy. It seems to be particularly bad on missions where you have to chase somebody.