I mean when they are for Windows only. Ever bought anything from Steam?
Well if they're Windows only then you'll need to run Windows on your Mac of course, I've only bought one game from steam - The Witcher (it was €2 lol). But I've downloaded a couple of free games from steam like Left 4 Dead, The Fortress etc. And I got Portal as a gift. But I don't really play that many "computer games" on my Mac. I've probably played over 200 hours of Diablo III though. And I also have Diablo II, Fallout and かたわ少女. But I mainly run emulators on my mac. And I have two monitors so I play games with my Playstation 3 Dual Shock-controller on "the big screen".