Amazing! I'm running Xgl under my Ubuntu- Linux Box, with only 10 minutes of setup!
For who don't known what is Xgl look these two videos
Novell's Video (with Gnome)
2nd Video (with Kde)
Xgl is a rewritten X-server based on OpenGL, all graphical effects are generated by the 3D-engine's card, so there isn't any cpu overhead.
For who want Xgl run on Ubuntu (and i think on Debian), read the guide on ubuntuguide.org.