PCI express is backwards and forwards compatible. You can run the latest PCIE 3.0 GPU in your PCIE 1.0 slot or a PCIE 1.0 GPU in a PCIE 3.0. The limit here is going to be your PCI bandwidth. It's going to be like running PCIE 2.0 at x8. It shouldn't bottleneck the mid-range GPUs too much, but it's going to really hurt the performance of the big powerhouse GPUs like the 680 and 7970.