Yeah, GTA IV doesn't run that great on any hardware configuration. The game needs more than 1GB of video RAM, and yet by default won't use more than 1GB due to some stupid programming decision. For optimal performance in GTA IV make sure you have at least patch 1.0.4.0, also turn down your draw distance and shadows, get your video memory usage down below 1GB, then it should be playable. The game will still dip down to around 30FPS in denser areas, but there is nothing you can really do about that. GTA IV was just that bad of a port.