Use a specific xorg driver
Normally xorg is automatically configured by HAL but perhaps you want to specify a specific graphic card driver. You probably have to do this after installing Nvidia or Ati binary drivers. Therefore just create the file /etc/X11/xorg.conf with the following content:
Section "Device" Identifier "Device0" Driver "nvidia" EndSection
Of course this example is for the Nvidia binary driver and you have to change the driver to the one you want to use.