Crossfiring options?

My freind is building a similar system to mine at the moment, and has two of the 3 6950s, but has a 6850 from his other build that he wonders if he can use in the system. It would run and all, PSU is fine, but would there be any improvement at all in performance, and also, what would happen if he tried to run it in crossfire?

You can't crossfire a 6950 with a 6970. This link (AMD's site) has a matrix showing crossfire compatibility. Bottom line is the cards must have the same GPU in them, which isn't the case here.

http://sites.amd.com/us/game/techn [...] firex.aspx

For situations where you pair up non identical cards (6950 and 6970 for example) I believe the 6970 basically steps down to match the lesser card's performance. For this reason it's a little rare to see mismatched cards because you will be wasting the extra performance of the better of the two. Cheers!