ERROR: You appear to be running an X server; please exit X before installing. For further details, please see the section INSTALLING THE NVIDIA DRIVER in the README available on the Linux driver download page at www.nvidia.com.
The error is pretty clear: you shouldn't install the drivers from inside X ("GUI").
So kill X:
ALT+F1- log in
sudo telinit 1
Now start the installation again. Read nVidia's Linux documentation, especially Before you Begin, for more info.