I don't know what class you are taking, but it sounds like you are being misguided.
XGL is an xserver that utilizes OpenGL.
Compiz is a window decorator that uses the OpenGL support of the Xserver to make windows and such wobbly. It can also make wobbly windows, etc using AIGLX which is an OpenGL implementation for the standard Xorg X server. It can also utilize the built-in support for OpenGL stuff in the beta Nvidia drivers.
Compiz uses EITHER:
Beta Nvidia drivers
And again, XGL has nothing to do with games at all, other than the fact that XGL uses OpenGL and so to games, but other than that, they are not related and have nothing to do with eachother.
Games work in Linux. Doom 3, Quake 4, UT2k5, soon to be UT2k7, ET:QW. It is not the fault of Linux that makes there be less games for it, it's the developers. The "emulator" you are talking about is Cedega, which can be found at www.transgaming.com
and the games that work in it, work fine. It doesnt slow your computer down. It's actually not even an emulator...