to disable compiz you have to start another window manager.
or back to compiz
so i expect you have used envyNG to install the ati driver.
I have not an ati card so i dont know if it has a configuration tool like Nvidia.
with xorg 7.3 it is ment to be self configuring so the xorg.conf file only holds input configuration, if that.
but i think you can still manually configure it, especially for complex configs.
you may have to google for a better config.
I saw someone recently run a command to enable external video output on their laptop, but i dont know what it was.
Now where was i going? Oh yes, crazy!