From here:

http://en.wikipedia.org/wiki/PCI_Express#PCI_Express_2.0

"PCIe 2.0 motherboard slots are backward compatible with PCIe v1.x. PCIe 2.0 cards have good backward compatibility, new PCIe 2.0 graphics cards are compatible with PCIe 1.1 motherboards, meaning that they will run on them using the available bandwidth of PCI Express 1.1. Overall, graphic cards or motherboards designed for v 2.0 will be able to work with the other being v 1.1 or v 1.0."