As you haven't installed the drivers yet, let's try that.
If you want the latest drivers add the repository to software sources:
sudo add-apt-repository ppa:graphics-drivers/ppa
sudo apt-get update
sudo apt-cache search nvidia | grep -oE "nvidia-[0-9]{1,3}"
I have a list like this:
nvidia-340
nvidia-346
nvidia-346
nvidia-346
nvidia-346
nvidia-346
nvidia-346
nvidia-346
nvidia-346
nvidia-352
nvidia-352
nvidia-355
nvidia-355
View full output here.
Then you need to install one of them. I currently have nvidia-355 (the latest at time of writing) active, but I have also got 352, 346 and 340 as well.
I'd suggest installing the latest non-dev version, which you can find by searching the nVidia site. This is 355 at the moment, so run this command:
sudo apt-get install nvidia-355
Then restart the computer when this completes. When it boots up, check everything is okay and login and then check the Additional Drivers window.
If anything looks buggy, use Ctrl + Alt + F2 to enter tty
and login. Then you can install an earlier version, with one of the following commands:
sudo apt-get install nvidia-331
sudo apt-get install nvidia-340
sudo apt-get install nvidia-346
sudo apt-get install nvidia-352
sudo apt-get install nvidia-355