Toms Hardware just did a 3 part series on crossfire/sli, dealing with different aspects.
Part 1, The Cards: Triple-GPU Scaling: AMD CrossFire Vs. Nvidia SLI
Part 2, The Slots: GeForce And Radeon On Intel's P67: PCIe Scaling Explored
Part 3, The Chipsets: P67, X58, And NF200: The Best Platform For CrossFire And SLI
#2, they take a look at a gtx 570 and a 6950 , single card
, performance in the various pci-x slots 4x 8x 16x.
Surprising results, some games show very little fps difference. Except there is one game F1, that shows pretty drastic differences.
This test is particularly pertinent to the recent crossfire/sli testing review done at Hocp, I will have to post this there to.
I would imagine, less than ideal results, especially in F1, with one card in x4 and the other in x16, but games like Crysis, would probably perform more like 8x-8x. I