HardOCP investigates the performance hit you take when plugging one of your SLI/CrossFireX cards into a PCI Express x8 slot instead of one with 16 lanes.
Have you wanted to space your SLI or CFX video cards farther apart on your motherboard to allow for better airflow? Do you have a motherboard that will not support a x16/x16 PCIe configuration? We put x16/x16 and x16/x8 PCIe SLI and CFX configurations head to head and show you what sacrifice there is to be made.
You can check out all the benchmarks over here. While there were measurable differences at insanely high graphics settings, the site concludes they did not notice differences in the gameplay experience between x16/x16 SLI or CFX compared to x16/x8 SLI or CFX.