This is, because the Folder /etx/X11 and all its contents are owned by root. When changing steeings, a backup of xorg.conf should be created and your current user has no rights to do it. Try executing this as a super user, like this:
sudo nvidia-settings
EDIT:
On Gnome you should use
gksudo nvidia-settings
On KDE you should use
kdesudo nvidia-settings