What Leaps-from-Shadows is saying, trying to, is that NONE of the current MOBO's support and NEVER will support PCIe 3.x. They are backwards compatible. Though I'd want it to be, PCIe 3 + GPUs 'today' won't be an added benefit. So unless 3/4-WAY SLI/CF is in the 'playbook' it won't make a difference. The GPU's today barely can saturate PCIe 2.x x8-lanes.
PCIe -> http://en.wikipedia.org/wiki/PCI_Express#PCI_Express_3.0
Yes, I'm certain the PCIe 3.x GPUs will work on PCIe 2.x slots and PCIe 2.x GPUs will work on PCIe 3.x slots; difference minimal. If only a single GPU then ZIP, no GPU current produced can saturate x16 PCIe 2.x or come close.
I'm waiting for LGA 2011 + SB-E.