I'd try lowering graphical settings while in the game and see how the graphics changes. There is a good chance that the low FPS is not graphics related slow downs. If you lower the settings with little change in performance, it may be a result of too much for the CPU, and nothing you can do will change it.
Xfire scales well & may give you 190% performance of a single 6950. However, there are still issues with microstuttering. It is a worthwhile upgrade, but waiting for the new 7000 AMDs and selling the old 6950 may be a better plan.
Yup, you should definitely be getting better stats for a 6950. I'm sporting a 6870 and even in high traffic areas with lots of effects I never drop below 35 FPS with all max settings. Granted, I'm at 1080.
Lots of people have been having FPS issues. I'd definitely wait to spring for another card when it might not make a difference.