From what I have gathered, the latest GPUs support PCIe 3.0 but are not using the full amount of bandwidth unless if they are in triple or quad SLI/Crossfire configuration. Also, do you only get full PCIe 3.0 support if you have an new Ivy Bridgte Intel chip and a motherboard that supports it? Does AMD have a chip that supports 3.0?
If a PCIe 3.0 card is inserted in a PCIe3.0 card, it has the max bandwidth. But still some say that it isn't 100% but you get extremely decent performance with it.
I haven't found any AMD chip with PCIe3.0 support. But Intel chip will give more performance in this case.