Just checked, 5850 is mostly around $300. Nothing low on ebay either. At least there isn't horrendous markup.
It's very nice though, since once overclocked, you can get 5870's stock performance, and they do overclock well.
The 5830 is priced right in between the 5770 and the 5850, but is a bit closer to the 5770 in performance from what I've read (take with a grain of salt since I've mostly been looking at the $300+ cards).
Also, if GTA is your main game, you should definitely NOT skimp on the cpu. Since the game utilized the cpu so much on the xbox due to its poor video card, the unoptimized port does the same with the computer.