I can get the GTX 280, GTX 280 FTW 2gb or the GTX 275 1792mb edition. Which one is better for gaming on an AMD 64 X2 4600+ with 8gigs of RAM on 1280x1024? I'm going for these cards as I want it to be useful for 2-4 years.
It's not that the 5850 will bottleneck a X2 4600+. It's the other way round.
Anyways, +1 5850, if you find one. In your case, you gotta upgrade your CPU, too.
BTW, check if the rest of your system is compatible, specially your PSU.