No difference.
PCIe 1 (or 1.1) has half the available bandwidth of PCIe 2.0. But most graphics cards don't come close to saturating the bandwidth of PCIe 1.1. Those that do are invariably dual-gpu graphics cards (HD5970, GTX295, HD4870X2).
A practical example would be Crossfire (2 graphics cards in 2 different PCIe x16 2.0 slots)....
On an X58 motherboard both cards run at full PCIe x16 , while...
on a P55 motherboard both cards run at PCIe x8 (each card running with half the bandwidth)
Yet for all practical purposes there is very little difference in performance with enthusiast grade graphics cards, even less so with mainstream graphics cards such as the GTX260.