Depends on what settings you want for your games. If you want medium-high settings at 1080, a 6850 is fine. But if you want the extra power and get high-ultra settings, the 6950 is the way to go. Just depends if the extra FPS/settings are worth that much to you.
Well I'm at 1600 x 900. Obviously the 6950 would be nice to have and would probably last a while but I also feel the 6850 seems like the more logical choice due to my lower resolution.