I deleted your post in the HD 7770 thread because the guy was talking about "CPU bottleneck" not "PCIe bancwidth bottleneck."
Even if it is a GTX 680/HD 7970, there is virtually nil performance difference between PCIe 2.0/PCIe 3.0 for games. Heck, even if you put GTX 680/HD 7970 in a PCIe 1.1 X16 slot, you'll see only ~3% performance difference in games.
Even if you put those cards in PCIe 2.0 X8 (half) or PCIe 3.0 X4 (quarter), you won't see any performance difference in games.
Now the reason why I deleted: What's the point of mentioning PCIe 2.1/3.0 etc. stuff to a newbie? Shall create more confusion and we don't really like confusion for newbie thread starters. And you said, "because HD 7770 is a low-end GPU, no difference." --> no difference in high-end as well.