xorg.conf does not exist in newer versions of grub anymore. However, if you want
you can create one using the following command:
"sudo Xorg -configure"

To run this go to virtual terminal "Ctrl+Alt+F1", and stop the gdm - "sudo
services gdm stop" (I think)
xorg.conf will be created in your home directory. Then you can copy it to
/etc/x11/
If xorg.conf is found in that directory kernel will use that. If it isn't found,
then kernel will use the default one. In case anyone mess up his xorg.conf file,
he just has to rename/move it to another directory and everything will return
back to normal.
Hope this helps.