yes. the only cpu series that even supports pci-e 3.0 is ivy bridge and sandy bridge-express so it wouldn't even matter. my 7850 runs on a pci-e 3.0 slot at 2.0 lane speeds due to using an i3-2120
as for the 6 pins, you should already have 2 6 pins and 1 6+2 pins. use any combination of the 2 to power up the gpu.