At this point, any version of linux would be decent for "gaming" providing you had the cedega package (commercial) that works. For the most part, you'll probably want either Suse or Mandrake, but stay away from Fedora if you plan on using it for a home system, otherwise you'll have to go through a bit more work than you should to play mp3 and other digital music formats :)
A OSX machine... that or A Windows box. realisticly linux hasn't hit mainstream gaming yet. At least not for the time and effort that is needed to put in to play games.
That's true to a minor degree, but not fully so.
Linux (through cedega) has the availability to play numerous games such as diablo 2 , World of Warcraft and a good number more (found at http://transgaming.org/gamesdb/) . Many of these games are fully playable, and a few have the unfortunate side affect of being not so playable. it just depends on the game really.
As far as a true gaming system in a true environment, yes, it's going to have to be windows (even macs suck in this department) until developers get off their tail ends and realize that the world wants games on a real platform, NOT windows only.
I have no affiliation with transgaming, cedega or any games developers, just found the application quite useful when looking for the same a few years back :)