When I used the Monitors utility, the desktop would look fine until I rebooted. Then I would lose the Panel. Searching for a solution produced no joy.
Then I read Dedoimedo's excellence analysis of why you must install the proprietary Nvidia drivers to make Mint work properly with your Nvidia card. See here: http://www.dedoimedo.com/computers/ubuntu-quetzal-nvidia.html
Dedoimedo's article will tell you why you need to install the Nvidia drivers - but the step-by-step instructions are for Ubuntu 12.10 and may not work for your monitor(s) and graphics card(s). Nevertheless, here they are. You can search the forum for more up-to-date instructions as needed.
(Comment:) I find this mildly annoying - another case of crap sliding downhill from Ubuntu.
How to install Nvidia drivers in Ubuntu 12.10
Updated: December 5, 2012
Step 2: Manually configure drivers
Rather than having to do this after your machine ends up with a half-working desktop, you can do this before any damage is caused. Hint, if you've already botched, then Ctrl + Alt + F1 to reach the first virtual console. Log in there, then follow the instructions.
In the past, running apt-get install build-essential was enough. This would get all the relevant stuff, including the kernel sources and kernel headers. Now, you must specify these separately.
sudo apt-get install build-essential linux-source linux-headers
If the transitional package linux-headers is not available, instead, please try running the command by specifying the exact version of the headers with:
sudo apt-get install linux-headers-`uname -r`
And this should work. Of course, it only magnifies the feces level of the latest Ubuntu release, but it could help you sort things out. Next, install the current Nvidia driver:
sudo apt-get install nvidia-current
If you skip the part where you install the sources, headers and the rest, you will see the following error message:
Processing triggers for initramfs-tools ...
update-initramfs: Generating /boot/initrd.img-3.5.0-17-generic
2012-10-30 13:37:19,611 WARNING: modinfo for module nvidia_current failed: ERROR: modinfo: could not find module nvidia_current
2012-10-30 13:37:19,611 WARNING: /sys/module/nvidia_current/drivers does not exist, cannot rebind nvidia_current driver
Once the installation completes successfully, check if the Nvidia driver is loaded:
sudo /sbin/lsmod | grep nvidia
If it's not, you will need to manually insert into memory. First, make sure all kernel module dependencies are satisfied and resolved:
sudo depmod -a
Then, load the driver:
This should succeed and the lsmod command should show it's loaded, but the usage count will probably be 0, because nothing will be using it yet. You will need to restart the graphics environment to actually benefit from the driver:
sudo /etc/init.d/lightdm restart
This will restart the graphics environment, you will see the Nvidia splash, and you will load into a desktop that has the nvidia driver running, and all that. Your problem is now resolved. For the skeptics among you, you can reboot to verify. Although there's really no need for that. Linux is flexible enough to setup drivers without having to restart the system.