Courtesy of Wiki "In August 2007, PCI-SIG announced that PCI Express 3.0 would carry a bit rate of 8 gigatransfers per second, and that it would be backwards compatible with existing PCIe implementations"
Also, keep in mind that a 3.x card will only run at the rate of the board slot. Your biggest worry is your PSU. IS it big enough? Enough amps on each rail? New enough so it has the correct power leads?