Make sure your /etc/apt/sources list contains non-free:

deb http://ftp2.de.debian.org/debian/ lenny main non-free contrib

aptitude update
aptitude install module-assistant fglrx-driver fglrx-kernel-src

Next you build your drivers:

module-assistant auto-install fglrx-kernel-src

Finally create a suitable /etc/X11/xorg.conf:

aticonfig -–initial

Reboot your system and login again.