Gaming on Leopard or Windows XP

Provided Windows has a much broader spectrum of games than Mac, I'd assume you'd mostly use XP for games; regarding games for both platforms, Mac would run them faster (that's just my guess, of course)

(You can't play Windows games on Leopard (unless you use emulators))

There would be no reason for the Mac to run a game faster than XP (provided you're talking about a game which has been developed for both platforms). XP video drivers tend to be more optimized than OS X video drivers and game developers who develop for both platforms tend to put a bit more effort into the XP versions as that is where their money is. And you can install Windows and run all of your Windows games natively on a Mac using BootCamp in Leopard. Gaming is less than ideal when using "emulators" (virtualization is the more accurate term... you are not actaully emulating anything). Running Leopards free BootCamp and using Windows via that method gives the best results for Windows gaming.