The 6850 doesn't really impress me; most tests are comparing it to the 5800s in terms of performance. I thought newer higher end cards were supposed to be better, but that's just me.
I love my GTX 460s (they're only $200 US now), but I'm using the EVGA Step Up program to get 470s now that they've dropped to $260 USD from the $310. I would strongly consider them as an option.
The HD 68xx cards are the successor to the 57xx series - it's a stupid change in numbers that AMD is doing. The HD 69xx will replace the HD 58xx. The HD 6850 is meant to beat the GTX 460 768MB (and it does).
The GTX 470 has some serious drawbacks with power and heat - it's only advantage is its CUDA performance (the 460 sacrificed CUDA for better power/heat/manif. cost).