... Who need a Mac Pro ? I own a iMac (standard without upgrade) 3.06GHz Dualcore - 4 GB DDR3, and 500 GB of disk space with a GeForce 9400 256MB, it's just sufficient. GTA SA does not need a High End CPU/GPU like Grand Theft Auto IV.
A Mac Pro is made for specific needs, is for those who work with the Adobe Creative Suite firstly.
I own a Mac and a PC, it's not i dont want to play on my PC but it will be cool to play on different platform so we can get (a little) more players.