I'd consider getting a 2nd 6850 for crossfire. You can get one on eBay for ~$100, and your resulting GPU power will be in the vicinity of a 7870 / 7950.
Couldn't find the Tom's US link, but here's the UK version. They have 2x6850 CF between the 7870 and 7950, with the caveat about microstuttering. http://www.tomshardware.co.uk/gaming-graphics-card-review,review-32501-5.html
I used to run 2x 6850's and it worked well. I replaced them with a 7950 because I needed the 6850's in 2 other computers.
i would not recommend 2 x 6850's at all. I ran them too, and the worked well in 50% of games, the other 50% was stuttering and FPS drops, things just werend smooth. I traded them for a single gtx660 and couldnt be happier now, smooth as butter in comparison to the 6850's. Didnt realise how bad the stuttering was with crossfire 6850's until i got the gtx660. would never go back to crossfire. Not to mention the 1gb vram of the 6850's is a limitation in many new games if you want to run high texture detail, which most people do.