gtx 460 is underrated and under priced imo. especially if you are going to bump the clock speeds up. $119.99 for a galaxy gtx 460 1gb 336, never used galaxy but i would go for that if trying to save money.
tboth cards trade blows in performance. one will perform better in some games, the other better in other games. The 6850 uses less power. Both overclock well. The 460 is a superseeded model and is no longer in manufacturer. So if you were looking at getting a second card for SLI at a later stage, it would be harder to track down.