You could potentially see benefits if you are running up against the 1GB of framebuffer. All of that GPU power you have can most definitely make use of higher settings, which in turn can be limited or hindered by 1GB of framebuffer.
Going to two 6950s could provide a smoother experience and might allow you to turn some settings up, like AA, in a few instances.
However if you don't notice any performance problems, like perhaps strange stuttering or low fps, then you most likely don't need to upgrade. But if you happen to turn up a few settings and your performance drops like a rock, then that could be a sign of the 1GB getting in your way.
Most games should be fine, but Crysis and Metro can make use of extra VRAM as they like to hog it.
I'm not seeing many sites test with higher levels of AA than 8xMSAA, so there's a chance if you use Supersampling the extra VRAM would be welcomed.