2) You'll get the most support probably on more popular variants like Ubuntu, Mint, Debian. I imagine any can be used as they are open source. You'll need to look up graphics card and sound card support to make sure that they work and are supported in whatever version of Linux you look at.
3) One thing I can add is that even if a game isn't on steam, you may still be able to play it under Linux. I know that WoW has a WINE-compatible version that I played for awhile on Linux Mint.