Best 6950 for Crossfire

There are 2 that probably stand out. The MSI R6950 Twin Frozr III is one of if not the best dual slot 6950 models. It's quite and cool giving you easy headroom to overclock.

The ASUS 6950 DCII is a 3 slot card with even better cooling, but 3 slots on a single card would make it tough to fit them in crossfire (although my motherboard can if I were to have a larger case).

The ASUS has a dual bios switch, the Twin Frozr III doesn't, but it's not require to have the bios switch to unlock the shaders.

Saved the money and dont get the twin frozr. Just unlock and oc past the twins specs. Always nice to have the backup bios switch just in case of a bad flash.

While the Twin Frozr III may be expensive (I haven't checked), the Twin Frozr II's are cheaper than the reference cards in many cases. The reference cards are a lot hotter and noisier than even the Twin Frozr II, and may not be desirable for crossfire.

Reference cards are getting harder to find as well. Right now there are none in stock on newegg (or there weren't any as of last night)

Twin Frozr IIs are still more expensive than ASUS DCuII, which has one of the best coolings, probably best for 6950s (and it's also a dual slot) is MSI Golden Edition, it's just awesome, but expensive... Also MSI has designed their cards for OCing, while it is easier (and safer) to unlock shaders on ASUS card and in most cases it gets higher clocks than MSI Twin Frozr II. Conclusion:
MSI Golden Edition>ASUS DCuII>MSI Twin Frozr III>MSI Twin Frozr II
I am not sure if Twin F. III is worse than ASUS DCuII in cooling, but i think that it might be better for overclocking.

Be aware about the ASUS DCuII, they make 2 versions, and one version takes 3 slots (the one with the really good cooling), the other takes 2.