Xgl is a Xserver architecture which uses OpenGL. It requires X drivers which are accelerated, such as fglrx. I used fglrx, although gnome-xgl-settings claims that 3D acceleration is disabled. However, that can be manually overided as per Using Xgl on SuSE Linux. After first `enabling desktop effects', everything is very slow indeed until compiz is used as the desktop manager. `compiz' is used automatically by Gnome, but various tweaks are required to get KDE to work with compiz, as described in Using Xgl on SuSE Linux. Even then, at the time of writing, compiz and KDE are not properly integrated.