Well take a look at this http://www.youtube.com/watch?v=IlX4o2ZXIKs.
6950 CrossfireX is on par with GTX 580. The GTX 580 is also on par with 7950. Crossfire 6950 should be on par with 7950 It comes down to your pick. Between price and preferences. However the 7950 is PCI-E 3.0 and future CPUs will support Pci-e 3.0 like the IVy Bridge. The 7950 is more future proof. How much are you planning to spend on the 6950?