For a long time after Debian Lenny came out I couldn't figure out at all how to install the ATi drivers, I always had one missing dependency or another, but now I know exactly what dependencies are required to use the ATi driver on Debian.
Download the ATi Driver
Code:
wget https://a248.e.akamai.net/f/674/9206/0/www2.ati.com/drivers/linux/ati-driver-installer-9-9-x86.x86_64.run
Log in as Root
Install Dependencies
Code:
aptitude install linux-kernel-headers-amd64
...