I've been reading around and most people have said that the GTX 660 will preform better in the games that I play (Starcraft 2, Diablo 3, World of Warcraft, Counter-Strike), but I found out that they were all talking about the 2GB GTX 660!
I'm looking for a graphics card that will let me play almost any game out there on at least medium settings without any trouble.
OEM only means it's the card straight from Nvidia or AMD, nothing tweaked or changed by the distributor (no custom cooling/backplates, factory OC, etc.).
But a 660 OEM shouldn't be significantly worse than a 7870, no. Pretty much it's up and down. 7870 outperforms 660 in some games, 660 outperforms in other. I would recommend going for a custom one though, instead of using a reference cooler. Getting something with a custom cooler can really help keep temps down, anywhere from 10-20C. And even sometimes a nice factory overclock.
Eh, 460W is kinda cutting it on the extreme bare minimum.. It's always good to leave some headroom. I'd say 600W at least, I'd get at least 800 if your gonna bother making the upgrade. 460W might work, might not.