This guide shows you how to install ati radeon drivers into a fresh debian install. First of all, you need to install from you repository mangager (Synaptic) this packeges:
fglrx-glx
fglrx-driver
fglrx-source
once you are done, you need to compile fglrx-source as says in fglrx-driver' s description:
This package provides 2D display drivers and hardware accelerated OpenGL for X.Org version 7.1. For 3D acceleration to work, you will need to compile the fglrx kernel module for your kernel: see the fglrx-source package. Note that the driver will work without the kernel module, but 3D acceleration will be disabled.
Open a root terminal in and type fast as you can:
m-a prepare
m-a update
and then compile it:
m-a a-i fglrx
If everthing works fine, try to load the your module:
m-a a-i fglrx
and if loads correctly, be sure that kernels loads it every time by adding this line into your /etc/modules:
nano /etc/modules
add this line:
fglrx
so it looks like:
# /etc/modules: kernel modules to load at boot time.
#
# This file contains the names of kernel modules that should be loaded
# at boot time, one per line. Lines beginning with "#" are ignored.
# Parameters can be specified after the module name.
loop
sbp2
vboxdrv
fglrx
okei. you are done! you just need to configure the xorg.conf to be able to use it in your windows manager. In the terminal write:
aticonfig --initial
now restart the X server pressing ctrl + alt + backspace and see if it works. After restart, type in terminal
fglrxinfo
my output is:
display: :0.0 screen: 0
OpenGL vendor string: ATI Technologies Inc.
OpenGL renderer string: ATI Mobility Radeon HD 3400 Series
OpenGL version string: 2.1.8304 Release
and it says that works! 
Hope it helps.