The only problem you face with PCI-E 3.0 GPUs is that they run in PCI-E 2.0 mode since MoBo PCI-E x16 slot is version 2.0.
GPU works just fine but it may not perform on it's performance level.
Further reading: http://www.hardwaresecrets.com/pci-express-3-0-vs-2-0-gaming-performance-gain/