GTA-IV Performance Thread
For me it runs like a dream on both my PC's. Install took around 30 mins on each. Benchmarks as below. Very smooth FPS so far no slowdowns at all. 180.48 with everything in the Nvidia control panel set to highest quality. No AA is used at all on either machine. View distance is quite high @ 1360x768 on my HDTV and everything else is maxed out. Lot less @ 1920x1200 with everything else maxed out but so far not noticing any pop-in at all so maybe with the high hardware of PC's (compared to consoles) this is not an issue now. Very noticeable that the blur filters used on the consoles to hide the lack of power to run this are not needed on the PC. Graphically the game does not look stunning but the scale of the city is still impressive and have not noticed any ingame city loading either yet.
Check out the CPU usage difference

More than double so looks like this is one of the first games which is better on I7

I'd say forget this game unless you have a quad core at the very minimum. Back in the early days of PC gaming it was quite common for the latest greatest game to require hardware upgrades.
I7-965 + GTX260-216 + 3GB DDR3-12800 Ram
Statistics
Average FPS: 39.72
Duration: 37.59 sec
CPU Usage: 32%
System memory usage: 78%
Video memory usage: 95%
Graphics Settings
Video Mode: 1920 x 1200 (60 Hz)
Texture Quality: High
Render Quality: High
View Distance: 28
Detail Distance: 100
Hardware
Microsoft® Windows Vista" Home Premium
Service Pack 1
Video Adapter: NVIDIA GeForce GTX 260
Video Driver version: 180.48
Audio Adapter: Speakers (Realtek High Definition Audio)
Intel(R) Core(TM) i7 CPU 965 @ 3.20GHz
File ID: benchmark.cli
Q9450 + GTX260-216 + 4GB DDR2-667 Ram
Statistics
Average FPS: 42.13
Duration: 37.32 sec
CPU Usage: 74%
System memory usage: 76%
Video memory usage: 88%
Graphics Settings
Video Mode: 1360 x 768 (60 Hz)
Texture Quality: High
Render Quality: High
View Distance: 53
Detail Distance: 100
Hardware
Microsoft® Windows Vista" Home Premium
Service Pack 1
Video Adapter: NVIDIA GeForce GTX 260
Video Driver version: 180.48
Audio Adapter: Speakers (3- ASUS Xonar D2X Audio Device)
Intel(R) Core(TM)2 Quad CPU Q9450 @ 2.66GHz
File ID: benchmark.cli