To be completely honest, I wouldn't consider running GTA IV on that hardware.
I have a 21.5" iMac, 3 GHz, 4GB RAM with the ATI 4670 card (256 memory).
Steam had GTA on sale for $13, so I picked it up, and it runs TERRIBLY. Has to be fairly low res, with low texture quality and it just destroys the game. It's playable, and relatively smooth, but the graphics are so muddy that it distracts from the whole experience.
Luckily I got a 360 for Christmas, and GTA runs great on that.
I hate to say it, but console gaming gives the best bang for your buck. ~$300 device and games run great, vs. a $1300 iMac which has to compromise for most gaming.