Post you /etc/X11/xorg.conf would help to resolve this.
Adding these entries if they do not exists should help.
Section "Module"
Load "glx"
Load "dri"
EndSection
Section "Extensions"
Option "Composite" "Disable"
EndSection
This is a good site for ati info.
http://www.phoronix.net/forums/forumdisplay.php?f=19
Copy of my xorg.conf.
# Xorg configuration created by system-config-display
Section "ServerLayout"
Identifier "aticonfig-Layout[all]"
Screen 0 "aticonfig-Screen[0]" 0 0
EndSection
Section "Files"
EndSection
Section "Module"
Load "glx"
Load "dri"
EndSection
Section "Monitor"
Identifier "aticonfig-Monitor[0]"
Option "VendorName" "ATI Proprietary Driver"
Option "ModelName" "Generic Autodetecting Monitor"
Option "DPMS" "true"
EndSection
Section "Monitor"
Identifier "aticonfig-Monitor[1]"
Option "VendorName" "ATI Proprietary Driver"
Option "ModelName" "Generic Autodetecting Monitor"
Option "DPMS" "true"
EndSection
Section "Device"
Identifier "aticonfig-Device[0]"
Driver "fglrx"
Option "DesktopSetup" "horizontal"
Option "OverlayOnCRTC2" "1"
BusID "PCI:1:0:0"
EndSection
Section "Device"
Identifier "aticonfig-Device[1]"
Driver "fglrx"
BusID "PCI:1:0:1"
Screen 1
EndSection
Section "Screen"
Identifier "aticonfig-Screen[0]"
Device "aticonfig-Device[0]"
Monitor "aticonfig-Monitor[0]"
DefaultDepth 24
SubSection "Display"
Viewport 0 0
Depth 24
EndSubSection
EndSection
Section "Screen"
Identifier "aticonfig-Screen[1]"
Device "aticonfig-Device[1]"
Monitor "aticonfig-Monitor[1]"
DefaultDepth 24
SubSection "Display"
Viewport 0 0
Depth 24
EndSubSection
EndSection
Section "DRI"
Group 0
Mode 0666
EndSection
Section "Extensions"
Option "Composite" "Disable"
EndSection
