I think the only time you might run into an issue is with a dual gpu card, but even then its highly doubtful
If you're only running 1 card at x16 pci-e 1.0(1.1) speed (which is around x8 2.0 bandwidth), you'll be fine. THG did some testing on this and there isn't a great deal of difference over x8 except in Microsoft Flight Simulator IIRC.