This tutorial will help you to install ATI drivers on Debian 5(lenny)
First you need to download ATI drivers from here and run the downloaded script as root
Follow the on screen instructions
If you got an error during the installation try the following:
#apt-get install build-essential module-assistant fglrx-driver fglrx-kernel-src
#m-a a-i fglrx
and then try to install the package again “sh ati-driver-installer-9-10-x86.x86_64.run”
It should be ok after this.