Whats your full system specs?(CPU,Mobo,PSU,etc.)
Whats your budget?
If your on a really tight budget then this 5850 is the best buy.It's performance to cost is right on target.It's faster than the 6850 and just a little slower than the GTX560 but costs $50 less.
http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=365429&CatId=3669