After activating the Restricted NVIDIA drivers at the Additional Drivers in the Hardware section in the control center, the NVIDIA applet in Mint will not write to the stock xorg.conf regardless of permissions set, and regardless of whether or not it's running as root.

 

Here's what I did to get around it:

 

1-Open the terminal

2-Type sudo mv /etc/X11/xorg.conf /etc/X11/xorg.conf.bak

3-Type sudo nvidia-xconfig

4-Type sudo nvidia-settings

 

The second step creates a backup of your currently working xorg.conf file.

Step 3 runs the NVIDIA utility to generate a new xorg.conf file that the utility can actually read.

Step 4 runs the graphical NVIDIA setup tool as root, so you can actually save your changes.

 

If this does not work then after step 2, do sudo rm /etc/X11/xorg.conf to delete your current xorg.conf file. Then run sudo nvidia-xconfig and sudo nvidia-settings.

 

Make sure that you have the latest driver version active.