I swopped some of my Quadro 4000 to the AMD7950
You get better results, but I cant remember the figures.
GTX 680 is a bit lower in performance, and as the AMD drivers are within the system it seems to work smoother
Also some rental companys here in germany switched to the 7950 from Qaudro 4000.
Problem is that you can only get them in the US or UK at the moment

I only had 1 X4 connected to it... Sorry...