Hardocp.com has a series of articles comparing the performance of SLI/Crossfire setups using x16 vs. x8 vs. x4 PCIe lanes:http://www.hardocp.com/article/2010/08/16/sli_cfx_pcie_bandwidth_perf_x16x16_vs_x16x8
"Gameplay performance can be summed up very easily and directly. We experienced no differences in the gameplay experience between x16/x16 SLI or CFX compared to x16/x8 SLI or CFX. The fact is you have to run at insanely high graphics settings just to detect the one to three frames per second differences we documented. With lesser resolutions and quality settings the difference in frame rates becomes so small that it is not detectable. Then in some instances we actually saw the x16/x8 configuration run faster."