Although there aren't as many games available for Mac as there are for Windows systems, there are still more than enough games to choose from. Here are some of the games I think are worth buying. I'm not listing it in any specific order. I'll try to update this list whenever I can.
TOCA Race Driver 3: One of the best all round driving games ever. Basically, its Gran Turismo for the Mac. So you can play this game for the rest of your life, and still not complete it.
F1 2012: The best and the most accurate Formula 1 game till date. Although F1 2013 is coming this year, F1 2012 is still a superb game, and the online experience is unparall