You should certainly append:
Section "Extensions"
Option "Composite"
EndSection
to "xorg.conf". Otherwise the composite extension does not get enabled.
Beware: "Composite" is case-sensitive!
In theory, is the minimal X config file supposed to support compiz?
On
my ATI R100 based laptop, this is not the case. Compiz says 'no sync
extension' and quits.
tjb