In order for these new cards to shine, you really need a motherboard with a PCIe 3.0 slot. And to take advantage of PCIe 3.0, you need one of the new 22nm Intel Ivy Bridge processors. The current Sandy bridge CPU's do not support PCIe 3.0.
Do you have a link / benchmark that shows the difference in gaming performance when running a 7970 on PCIE3 vs PCIE2. I thought even the top prior generation cards showed little difference between PCIE2 and PCIE 1 unless using SLI. I find it odd that the prior generation had more then enough bandwidth but that the new card is bottlnecked by 16 PCIE2 lanes.