If you’re lucky enough to have a dual headed video card and an extra monitor lying around, there are a few options for getting a dual-monitor setup working in X Windows.
If you have a dual-headed nVidia or ATI card, the binary-only TwinView and BigDesktop drivers (respectively) will probably perform well, but if you don’t fall into either of these camps, there are still options available.
Xinerama is a standard X extension which will work with just about anything, but you give up that ability to do OpenGL direct rendering on the second screen. For many uses, this may not even be an issue. If you need the 3d support on both monitors, however, a subset of open source drivers are supported by the MergedFB driver, which does support 3D direct rendering on both screens.
Check out the dual monitor howto on the Ubuntu Forums. It covers all four options and should be relevant for any current Linux distro –Link.