aticonfig --initial te lo sconsiglio
a me aveva azzerato il file xorg.conf
se vuoi ti posso mettere il mio file xorg.conf come riferimento
scheda video ATI9600 monitor Dell 19" LCD
driver ATI 8.5
accelerazione 3D e compiz-fusion funzionanti
# xorg.conf (X.Org X Window System server configuration file)
#
# This file was generated by dexconf, the Debian X Configuration tool, using
# values from the debconf database.
#
# Edit this file with caution, and see the xorg.conf manual page.
# (Type "man xorg.conf" at the shell prompt.)
#
# This file is automatically updated on xserver-xorg package upgrades *only*
# if it has not been modified since the last upgrade of the xserver-xorg
# package.
#
# If you have edited this file but would like it to be automatically updated
# again, run the following command:
# sudo dpkg-reconfigure -phigh xserver-xorg
Section "Files"
Fontpath "/usr/share/fonts/X11/misc"
Fontpath "/usr/share/fonts/X11/100dpi/:unscaled"
Fontpath "/usr/share/fonts/X11/75dpi/:unscaled"
Fontpath "/usr/share/fonts/X11/Type1"
Fontpath "/usr/share/fonts/X11/100dpi"
Fontpath "/usr/share/fonts/X11/75dpi"
Fontpath "/var/lib/defoma/x-ttcidfont-conf.d/dirs/TrueType"
EndSection
Section "Module"
Load "bitmap"
Load "ddc"
Load "dri"
Load "extmod"
Load "freetype"
Load "int10"
Load "vbe"
Load "glx"
EndSection
Section "InputDevice"
Identifier "Generic Keyboard"
Driver "kbd"
Option "XkbRules" "xorg"
Option "XkbModel" "pc105"
Option "XkbLayout" "it"
EndSection
Section "InputDevice"
Identifier "Configured Mouse"
Driver "mouse"
Option "Emulate3Buttons" "true"
EndSection
Section "Device"
Identifier "ati"
Driver "fglrx"
Option "VideoOverlay" "on"
Option "OpenGLOverlay" "off"
Busid "PCI:1:0:0"
EndSection
Section "Monitor"
Identifier "dell"
Option "DPMS"
Horizsync 30-81
Vertrefresh 56-76
EndSection
Section "Screen"
Identifier "Default Screen"
Device "ati"
Monitor "dell"
Defaultdepth 24
SubSection "Display"
Option "AllowGLXWithComposite" "On"
Depth 1
Modes "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 4
Modes "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 8
Modes "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 15
Modes "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 16
Modes "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 24
Modes "1280x1024" "1024x768" "800x600" "640x480"
EndSubSection
EndSection
Section "Extensions"
Option "Composite" "Enable"
EndSection
Section "ServerFlags"
Option "AIGLX" "true"
EndSection
Section "DRI"
Mode 0666
EndSection
sarebbe interessante cpntrollare anche i log /var/log/xorg.0.log
potresti provare a entrare senza X
e correggre eventuali errori nel file /etc/X11/xorg.conf

tu come li hai installati ?
se non ricordo male mi pare di aver fatto così
sh ati-driver-installer-8-5-x86.x86_64.run --buildpkg Debian/lenny
dpkg -i fglrx-*.deb
sudo module-assistant prepare
sudo module-assistant update
sudo module-assistant a-i fglrx