If you have UT2004, download this:
http://www.macupdate.com/app/mac/15039/santaduck-toolpak-for-ut2004
And you can run benchmarks that way.
Quake 3, Doom 3, Quake 4, Halo, and probably others have built in benchmarks and timedemos, if you Google around with the game name + benchmark/timedemo you should find instructions, it's all pretty easy.