This link may help you, if you have problem installing the ATI proprietary driver:
http://wiki.debian.org/ATIProprietary

Looks like the non-free repo's package for the newest "stable" version of Debian (6.0 Squeeze) removed support for your video card's chipset. But if you're running version 5.0 Lenny you'll be good to go. Otherwise you may find yourself download and installing in a more manual way.