Apparently, a single 6950 would even outperform 3 450gts cards on a tri SLI configuration, although that's just what I heard. Could easily be true, considering that having multiple cards on SLI/xfire doesn't actually double/triple/etc the performance. And 6900 series cards are known to have much better scalability, so they're ideal if you plan on xfiring them in the future. Just make sure your mobo is "crossfire-ready" too. *
You might also be wondering if there's a difference between a 1gb 6950 and a 2gb one. There are reports that, at 1920x1080, BF3 (maxed out, ofc) would use more than 1gb vram. This is probably where having a 1.5GB card would've been useful. But actually, I have friends who run the game fine on ultra with a 1gb card (~60fps). Still you'd have to keep in mind there may be games that eat up more vram in the future, and the 2gb version of the 6950 doesn't really cost much more, assuming you'd buy one of these.
* Edit: nvm, forgot your mobo actually supports both.