There are official Debian packages containing the proprietary ATI RADEON and FireGL display driver: http://packages.qa.debian.org/f/fglrx-driver.html. Follow the instructions you can find in the README.Debian file(s) shippped with these packages.
Obsolete installer packages
About the obsolete installer packages: As of 2006-04-18, Flavio's [http://www.stanchina.net/~flavio/debian/fglrx-installer.html site] says "These packages are hopelessly out of date and I don't think I'll have time to update them in the near future, because I believe it's more useful to work on the official Debian packages."
[http://wizah.blogspot.com/2006/10/debian-how-to-ati-drivers.html Debian HOW-TO : ATI drivers] explains how to install and configure the latest ATI proprietary drivers using module-assistant and the official ATI installer.
Be aware that upgrading from or to these packages (created by the ATI installer) is and will most probably NEVER be supported by the Debian maintainer of the [http://packages.qa.debian.org/f/fglrx-driver.html fglrx-driver] package(s)!