Actually a well-chosen 6850 will overclock to 1000mhz/1200mhz performing above stock 6870 levels:
And the 6870 stock compares very well to an overclocked GTX460:
The GTX 460 used is reviewed here (765/925/1530):
The 6850 uses LESS power than a 460, and only needs 1x 6pin plug so it's easier on your PSU and compatible with more PSU's.
And if you have an AMD system capable of crossfire, the 6850's overclocked and crossfired will perform closer to GTX470 SLI than GTX460 SLI, besting a the 5970 in most situations. Crossfire is easier than performing an SLI hack on older motherboards, and is more commonplace in the AMD lineup.
All things considered, I think you should get whatever is cheaper, but if you have any potential and desire to crossfire or SLI later on, then you should get a 6850, undoubtedly. I'm super stoked on my 6850 frankly - I've been trying to justify getting a 2nd to crossfire but I just don't feel that I really need it, and I run @ 1920x1080. It is stable and runs fairly cool (<70degreesC) @ 1030mhz/1172mhz w/ 1.25 volts but I'm fine with performance @ 962mhz/1162mhz w/1.174 volts
Either a GTX460 or a 6850 will be wonderful, but all in all, and for an AMD user, I think that the 6850 is actually the better choice. Of course F@H, CUDA or Physx usage lends to the 460, but Eyefinity and power/performance ratio is to the 6850.