7770. Most of the Catalyst drivers released lately as well as most likely in the future inputs great focus on the Radeon HD 7xxxx series. A 7770 performs the same, if not slightly better than the 6850 using considerably less power.
Here is review where you can compare a 650ti to a 7770 (the main 65oti in te article is an oc one so outside of budget)
The $120 6850, the 7770 and the 650ti are all really close in terms of price/perf with the 6850 gaining at higher resolutions (where all the cards ar unplayable anyways).
I would not grab the 6850 since its betweent the two cards in pricing and perf but will less likey have less driver priority and uses more power. It would be useful in cf for higher resolutions due to its higher bandwidth.
I would grab the 7770 for the cheapest option, 650ti for best performance and the 6850 only if you don't mind the heat/power andreduced driver support (eventually). The 6850 has the best price/perf though if you don't count MIR (which has mixed results and requires a mailing cost) so take that into consideration.