Debian Installation Guide
2007-11, see http://wiki.debian.org/AtiHowTo
As of November 2005, ATI's drivers are in the non-free area of Debian. Make sure your /etc/apt/sources.list contains "contrib non-free" as well as main. You will, however, have to build your own kernel modules.
Warning: do not mix the non-free packages with the packages created by the installer as they will conflict in non-obvious ways. Before switching from one method to another, completely remove all fglrx-related packages. Never do upgrade from one method to another directly!