Personally, I would go for the 6850, or if your budget can stretch just a little bit, go for a 6870. Make sure that your PSU is up to the job though (6850 needs 500w with 1 6 pin pcie, 6870 needs 500w but with 2 6 pin pcie)
I hope this info is useful!
The HD 6850 and GTX 460 1GB are equal in terms of performance, the one card scores 1-2 fps more on one game, the other one 1-2 fps more on other game. The GTX 460's stay cool and are more overclockable but the HD 6850 is 20-30 $ cheaper. (i'm talking 'bout 1GB GTX460's, the 768MB versions are 10-20$ cheaper than the HD 6850 but can be a lot slower @ 1080p with AA on)
So this might be a personal preference of brands after all. And you have the HD 6870 @ 220 $ (the GTX 460's 1GB starts @ 210$), which might also be a option for you.