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.
I should also note that I use this machine to edit HD video as well, so far no issues but have used newer better cameras the video bit rates are getting higher. I was hoping adding another 5770 would be overkill and I won't have to worry about video cards for a couple years.
However after reading about the 6850 and it isn't much more money if I should just go that route.