The drivers and extreme noise levels for a pair of 6990's in Crossfire make it highly likely that they would be a disappointment. You should read the threads about AMD drivers before purchasing a pair of Crossfired/quad GPU 6990's.
http://forums.guru3d.com/forumdisplay.php?f=46
http://forums.guru3d.com/showthread.php?t=341660
http://www.tomshardware.com/forum/313079-33-tomshardware
Also check out this article. Multi-GPU's don't even kick in on some of the games:
http://www.guru3d.com/article/triple-monitor-gaming-on-geforce-gtx-590-and-radeon-6990/
"Weirdly enough though, how do I say this right, I felt that the R6990 is the stronger performer, but the GTX 590 is simply better supported and runs a tad smoother at 5760x1200. In terms of heat and noise the GTX 590 will be the clear winner. And as stated, performance wise the R6990 is the frontrunner. Now then, the dark horse -- Quad GPU gaming with two cards is in favor for NVIDIA, the overall scaling is much better, and here again each team takes some wins and total losses."