I think it's also worth mentioning that if you use the Nvidia driver,this will also create an xorg.conf for you.
If you run the install script from Nvidia,it will ask you during the installation if you want to let it edit the xorg.conf for you.
I normally just let it do it.Here's an exert from mine:
ade@Pc1:~$ cat /etc/X11/xorg.conf
# nvidia-xconfig: X configuration file generated by nvidia-xconfig
# nvidia-xconfig: version 1.0 (buildmeister@builder58) Fri Aug 14 18:33:37 PDT 2009