Yoooo guys! I've been experiencing my own problems with my new crossfire config (2x 6950's) and I must say, its quite complicated getting it all ready to run. Here's some tips I encountered while troubleshooting my both cards:
Turning on AFR ( Alternate Frame Rendering) can increase fps in most games up to about 50% (in my case)
DOWNLOAD THE "CAP" from the amd driver website, these are absolutely VITAL for gaming in battle field 3, I was experiencing issues where I had been running out of memory ( 1gb 6950+2gb 6950) where I used to use about 1300-1500mb on ultra (without MSAA), downloading the cap from amd has improved this, now I run on about 800-900mb memory usage, with ultra settings (no msaa, but can be turned on with no issues).
Turn off ULPS (Ultra low power saving) in the registry if you're experiencing BSODS when gaming or opening msi afterburner (google on how to disable ULPS)
I've noticed, if you run out of memory on your video card, and use your hard drive (lag spikes) it can cause BSODS aswell! Watch out, I had a few of these bsods when I didn't have the CAP installed.
Last but not least, if you're going for overclocking but aswell stabilty, I recommend not overclocking too much in crossfire, I used to run my single 6950 on 900 core, it isn't possible in crossfire. I run both cards on 840/1375.
Didn't experience any bsods since then!
Hope this has helped some of you, would love to hear feedback!