That 520w Corsair PSU is more than enough for what you listed.
if you wish to add another 460 in the future for SLI
Not possible on the Gigabyte GA P55-UD3R.
then you will certainly want to upgrade the PSU to at least a 750w or greater with a high value on a single 12v rail...
Not true, you can easily do two 460s on a good 650W PSU, such as the XFX 650W XXX Edition, Corsair HX 650, Antec Truepower New TP-650, NZXT Hale90 650W to name just four examples, and they all have the necessary number of PCI-E connector cables. And a single 12V rail is certaintly not any kind of requirement and it has no advantages over a good dual or quad rail PSU.