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Prepare your system
First you need to install the following packages
#apt-get install module-assistant fglrx-driver fglrx-kernel-src

Build and install module
module-assistant auto-install fglrx-kernel-src

Update xorg.conf file
Use the following command to Update xorg.conf file:
#aticonfig --initial

After completing configuration you need to restart your system.
You can test to see if the restricted driver using the following command:
fglrxinfo

Output looks similar to the below

display: :0.0  screen: 0
OpenGL vendor string: ATI Technologies Inc.
OpenGL renderer string: ATI Radeon X1200 Series
OpenGL version string: 2.1.8304 Release

Note: If you get an update of the kernel from say linux-image-2.6.26-2-686 to linux-image-2.6.26-3-686 it may bork the drivers, which would need uninstalling the drivers and recompiling of the drivers.