CrossfireX or one High power card?

typically 1 good card is recommended over sli/xfire as not all games will function properly or use dual cards.

I would go for a 2nd 6850, since that card is already decent as it is. The way I see it, I get 1 pretty fast card, few years later get an 2nd card run in Crossfire or SLI for cheaper. 6850 are pretty cheap at the moment, Get another 6850, it will perform close to a 6990, Not quite but much faster then a single 6970. If you got the money get a 7950 or a 7970, or a GTX670/680. Your choice. But remember, all games don't support crossfire, If not set the Crossfire option to AFR Friendly to force 1 card to render even frames and the other odd frames.