I had 2 6950's, 2 GB ref, unlocked...
And bought 2 x gtx 560 Ti Phantom...
I hated my 6950's:
Noise ( yes, A LOT of noise )
Temps ( over 90 without custom profile )
I <3 my GTX 560 Ti Phantoms
( 2 GB each )
-Quiet, VERY quiet, I can't hear them ( latterly, the noise trough the window is more and I'm not in a city )
Even in heaven , case fans all on low and H100 intake, barely audible
-Temps: Sub 75 all the time stock profile, in SLI
-overclocking: running 950/1002... EDIT: that's a skech oc xD, didn't put work in it yet
performance: about equal, or a bit less... but It's worth it, get the GTX 560 Ti's...
Get GTX 560 Ti's, if possible: the Phantom from gainward, with 2 GB... This card is worth every penny twice
I hope I can help with this info
One more thing:
The GTX'es are wayyy smaller, check out this
And they look sooo sexy in this system