GTA IV is an un-optimized piece of...uh...poo. Anyway, it seems like you may want a better video card, maybe one with +1GB of VRAM (GDDR5 is the fastest). Try getting 4GB of RAM. The processor is iffy, but leave it to see if there is still an issue after getting a new video card. The game is not just GPU based, but CPU based as well. You are going to need a lot of CPU power to run that game.
Well first your CPU is fine, however your graphics card is not ment for gaming.
Also GTA 4 has many settings. for low end systems i would recommend you keep the shadows turned off as they can be very intesive on the systems performance.
Also turn down the draw distances to about 15-17 that should help boost performance.