As can be seen HERE, a 7970 is benchmarked at various lane bandwidths on PCI 3.0.
As PCI-e 3.0 8x has the same bandwidth as PCI-e 2.0 16x you can see there is no benefit to going to PCI-e 3.0 (small fluctuations could be margin or error)
Hell as you can see some don't even benefit from using 16x 2.0 over 8x 2.0
Plus the PCI-e 3.0 on the ASUS board is being provided via a PLX chip rather than natively through the 990fx chipset. Meaning it could increase lag or micro stutter as data is passed to the GPU.
Unless you are fully loading all the PCI-e Lanes, PCI-e 2.0 is still good for single or double GPU usage.
thanx, really helped me. going for the asus sabertooth r2.0. and i don't want intel cuz i have a i5 and it sucks. can't do a lot at the same time:). and i go for crossfire in the future and when my gpu get cheap and i need more performance maybe quad:)