Let say it cost a little less and is 10% performance under 580gtx at 150w less tdp.
Now, a 6990 under such tdp, going to totally rock.
single card performance, is though to do when the chips get big, small efficient and fast, the crossfire performance with the 6850 has been phenomenal.

there been more rumors, and I read Gibbo as a super great card for the price/performance ratio, which 90%+ people buy.