i think that if ur mobo doesnt support pci 3.0, it wont work as far as i know. i asked a friend of mine who knows his stuff on computers, and said that if a GPU works on pci 3.0, and ur mobo has 2.0, ull have to change the mobo to make the GPU work. im not sure, im just saying what i was told. (if im wrong, please correct me, guys.)
on the GPU part, maybe u should get a geforce gtx 660
if u have the money... if not, i think an amd radeon which i believe are cheaper, should work great also
one last thing, that 460W PSU is a joke if ur into playing games and adding components to ur PC in general. id change it to something more powerful. The minimal power supply rating for a GTX 660 is 450W i think.. to be on the safe side, and to be sure the pc gets enough power to power everything well (GPU included), id get a more powerful PSU.
my computer came with a 450W PSU when i bought it, and now that i began adding accesories to it, like a geforce gtx 550 ti (cuz my mobo supports PCI 2.0 and it was the best i could buy at the moment) and auxiliary fans with LEDs, it began to restart randomly while gaming and the psu was getting hot (not normal). i changed it for a Coolermaster PSU rated at 600W and it works just fine with no PSU getting hot :P
hope that helps :P