First off, what kind of games are you looking forward to play? Don't tell me Black Ops because my friends HD 4650 can play that smoothly.
Apparently the 6850 CF scaling is VERY good, comparable to the GTX 460 SLI. It is definitely one of the most bang for bucks setup you can get. Hd 5770 CF still does well, but if you're looking for something that is future proof and "can run Crysis", then 6850 CF is the way to go.