Generally though you can get any game you want.
I know there is a website that specializes in Mac games but I totally forgot what it was called.
Are the PC versions playable or do I need a special game?
This is actually an interesting question, in that while PC versions of games are not playable on Macs unless you install Windows via bootcamp, there is a small amount of hybrid disks with both PC and Mac versions. This is mostly limited to the Blizzard family of games, like the Warcraft and STarcraft games.
As well most Mac games still come out months after the PC version of the game. I'm still waiting to try out UT3, which hasn't come out on the Mac.