I just asked a similar question of EVGA because I have an older board from them that offers 2.0 support. I'm looking at two 6950's too.
I am looking to upgrade for BF3 and wondering about compatibility.
1) Can I use two 6950's in crossfire?
2) They seem to be pci express 2.1 (along with every other card out there) and the 790i ultra appears to do 2.0 only. What does that mean?
I am also hoping to add a vertex 3 120gb with windows 7 64 to make better use of the 4gb of ram I have as vista 32 doesn't seem to use 4gb as effectively.
Yes you should be able to run two of those in Crossfire without issue. PCI-E 2.1 implements some power management and troubleshooting features of PCI-E 3.0, but still has the same bandwidth of PCI-E 2.0. You will not see a loss of performance using them in the PCI-E 2.0 slots. Upgrading to a 64-bit OS would allow you the full use of your 4GB of memory. Please let us know if you have any further questions.