You need to play around with the graphics settings. Perhaps they are simply too high.
Set everything to medium settings. Move sliders to the middle of the lowest value and highest value. Doing to should make the game playable to simply load up a saved games or start a new game to verify performance is fine. Then start increasing some of the settings, but don't max out any of the sliders; especially draw distance.
I had no problems playing GTA IV on my Q9450, Radeon HD 5850 1GB, and 1920x1200 resolution monitor. Most of the settings were on high like textures, water reflection; can't remember the rest. All sliders were somewhere in between "medium" and "max", but closer to medium.