I'm suprised that you can even find PC games in retail outlets. In the UK it's all PS3, Wii or XBox360.
For Mac games, keep a close watch on previews and reviews of offerings at some of the Mac Games sites, ie.
http://www.insidemacgames.com/ or in your Mac magazines.
Then check for downloadable (or cover CD) demos. That way you avoid most of the bad games and buy the stuff you like.
Then buy from Amazon. Downloadable games are great, but nothing beats having the physical CD / DVD in your hand.
Avoid EA games, the anti-piracy measures are extremely annoying to those that actually buy the games. Only the pirates have ways round their anti-piracy measures, so ironically, they enjoy the game a lot more than if they'd paid for it.