Unable to remove old X config backup file /etc/X11/xorg.conf.backup.
Many people complains me that they get error "Unable to remove old X config backup file '/etc/X11/xorg.conf.backup'." after pressing "Save to X configuration file" in nvidia-settings. The reason is very simple, launching nvidia-settings doesn't give you enough permission to write inside /etc/. So run nvidia-settings in the following way:
type gksu nvidia-settings