All Phenoms exhibit some slight degree of inherent bottlenecking (lower average/peak framerates compared to i5/i7), but, the same degree of improvement (50-80% boost) in framerates should occur with the adding of/successful Crossfiring of a 2nd 6950....
Make sure its a decent power supply brand such as corsair. Its really hard to tell what the difference in fps will be. But your cpu is getting dated. First buy the gpu then when you have the cash, get a new cpu.
This is at lower res. but increasing it puts more work on the GPU than the CPU also you are just over the 3.5GHz so you can get a bit more:
There is no 1080 bench mark or dual 6950 but the dual 6970 at 1680 x 1050 will be close see:
So your CPU can do around 35 min FPS 50 Avg FPS and 2 x 6950s can do around 48 min 69 avg and a single one around half that. In conclusion your CPU will bottleneck a second 6950 to run around half way between a single and dual cards in Crysis 2 at 1920 x 1080 based on approximate data.
BTW I have exactly the same CPU at the same clock and unlock.