Check out this post

http://www.neogaf.com/forum/showthread.php?t=344018

I thought for a minute maybe this game is optimized for Nvidia, so I went and took out my HD 4870 1024Mb and replaced it with my brothers 8800GT (180.43 beta drivers) and to my surprise, performance still sucks ass.

Sorry Guys it seems that it does not matter what GPU you got, performance is still gonna suck.

**** THIS.

A few weeks ago we got the GTA 4 requirements and to my suprise a Quad Core was the recommended CPU. Ok No problem, so I upgraded from my old but powerfull Core 2 Duo E6600 2.4 Ghz to get me a nice new shiney Q9300. With that, a new motherboard and a new graphics card came as well, because IMO GTA 4 was the most anticipated PC Game this year (especially us guys with no consoles) and i really wanted to enjoy the PC version in all its glory, but THE ****ING PERFORMANCE SUCKS ASS. I regret buying this damn game.

Running at 1680 X 1050 with everything on low and i cant breach the 45 FPS mark.

The framerate is dipping below the 20 FPS mark much too frequently, im getting like an average of 25 FPS. Did Rockstar just drop a stupid unoptimized port or is this a nightmare.

My System:

MOBO: Asus P5Q-DLX

CPU: Core 2 Q9300 @ 2.83GHz

GFX: Saphire HD 4870 1024MB (Cat 8.11)

MEMORY: 4 GIGS (2x2Gigs Patriot Extreme PC9600)

Power : 850W

I Recommend You do not get this game untill a patch is out to fix up the ****ed up performance or buy an Xbox 360 or PS3 and play it on that.