Also worth bookmarking:
http://www.apple.com/games/ - I still can't understand why Apple make this section of their site so tucked away and hidden!
(1) What are the *BEST* FPS games available for OS X?
Unreal Tournament is still my favourite. Quake and Doom series are great too.
(2) What are the *BEST* RPG games available on OS X?
I don't play them much these days, but World of Warcraft is pretty much dominating in the MMORPG arena at the moment. There are plenty of others to look at though.