Hi, sorry for not showing up sooner...
First things first : does direct rendering work? "glxinfo | grep direct"
Are you trying to use Compiz or the Metacity integrated compositor?
You might try specifically enabling AIGLX; the composite extension and checking that you have access rights to the DRI device in xorg .conf
Code:
Section "DRI"
Group video
Mode 0666
EndSection
Section "Extensions"
Option "Composite" "1"
EndSection
And in the ServerLayout section add "Option 'AIGLX' '1' "
Also check that you installed the correct driver version for your card.