And Here again. NOTE its RE the 7970 but as performance is quite close so is the bandwidth limiations.
"QUOTE:Ultimately what is clear is that 8GB/sec of bandwidth, either in the form of PCIe 2 x16 or PCIe 3 x8, will be necessary to completely feed the 7970. 16GB/sec (PCIe 3 x16) appears to be overkill for a single card at this time, and 4GB/sec or 2GB/sec will bottleneck the 7970 depending on the game. The good news is that even at 2GB/sec the bottlenecking is rather limited, and based on our selection of benchmarks it looks like a handful of games will be bottlenecked. Still, there’s a good argument here that 7970CF owners are going to want a PCIe 3 system to avoid bottlenecking their cards – in fact this may be the greatest benefit of PCIe 3 right now, as it should provide enough bandwidth to make an x8/x8 configuration every bit as fast as an x16/x16 configuration, allowing for maximum GPU performance with Intel’s mainstream CPUs."http://www.anandtech.com/show/5458/the-radeon-hd-7970-reprise-pcie-bandwidth-overclocking-and-msaa
In summary there would be no improvement for getting a PCIe 3.0 compatible mobo unless your looking to SLI 680's or getting a 690 when it's out. I would stick with the P67 and get the 680 if it comes time that u want to SLI then look at getting a PCIe 3.0 board and IVY bridge CPU if u feel the need.