Long story short, I have 2 Radeon 6850's running in Crossfire. When they run together, I receive very low performance on Skyrim. With only one card running (I disable Crossfire) and I max everything, including AA and FXAA (x8 and x16 /w the FXAA box checked) and i recieve 50+ fps. Normal?
AMD has released a follow up 11.11b and 11.11c set of drivers to try to address the issues. Personally I run 5850's in crossfire and am still having problems with the game. Just a very poorly coordinated pc launch. The other platforms have had problems as well, but things are more complex on PC. I've had to use a third party .dll file to resolve the performance issues. I run at 2560x1600 and on Ultra there is a problem that causes the game to slow to a crawl. Alt-tabbing gets it going, but it sometimes lapses back to the super low framerates well below single card performance.
I'd honestly recommend just using 1 card if you're running into a ton of issues-especially if you can run at your target resolution/settings. I hope they resolve this stuff but after a couple of weeks I'm not exactly holding my breath.
Playing Skyrim on a I-5 750 @ 3.8ghz with 8 Gig RAM and using a HIS 4890 I was getting a solid 28-32 fps at high settings. Added a second HIS 4890 and my FPS DROPPED 10+ FPS to 15-17 FPS. I added the AMD profiles and installed Radeon Pro using the settings I found on this guide
I didn't use the Driver Sweeper or CCleaner and I didn't remove my old drivers like the guide suggested. I just added the aforementioned profiles and the Radeon Pro. This UPPED my FPS to a Vsync locked 60 FPS, so I upped my settings to ultra and I'm getting 38-52 FPS depending on where I am in the game; with the lowest FPS in big cities. I can't say exactly why or how this worked, since I added both the profiles and Radeon Pro at the same time, so I don't know if it takes both to work, or just one or the other; but it did work. I'm happy with it.
I'd suggest trying the Anti-Aliasing Mode set to Adaptive(Performance) instead of the lower setting Multi-Setting. It does look much better and only drops FPS a few frames. Setting to the highest setting of Adaptive(Quality) drops FPS drastically, in my case to 28-38 FPS, and in my opinion doesn't look that much better.
Hope this helps. Just FYI the Crossfired 4890s are performing better than or equal to a 6970/5970 ( around $350 ) or a GTX 570 (around $340 ). You can get a pair off Ebay for around $100-$120.