Bungie makes all the best games for Mac, like Marathon, Myth, and the awesome upcoming game HALO
Excluding the few games that come as Mac/PC games on release (Blizzard games, the Sims and a few others) most of the Mac versions of games are pretty expensive compared to their PC counterparts. It's probably cheapest to spring for an OEM copy of Windows [http://www.zipzoomfly.com/jsp/ProductDetail.jsp?ProductCode=10011180] and run bootcamp if you plan to buy more than a couple games for your Mac.
That, or wait 'til Valve finishes porting Source and Steam.