running E8500 @4.2 GHz, 4 GB RAM, 4870 512, WIN XP
playing at 16*10, med textures, highest rendering, detail settings 20-40
I'd estimate a FPS average of 30. game probably runs between 20 FPS to 40 FPS while driving through the city. when out of the car, game seems to run above 30 FPS. when in doors, no FPS issues.
please note I'm running the game with Vsync on since I think it helps smooth out the game by making FPS variance less noticeable.
I should add that if you do pick up GTA 4, then I'm pretty sure you are going to have to play at a low resolution and textures due to the 256 vid RAM. that's assuming you get playable framerates at the lowered settings.