If the NVidia installer says X is running, then you should believe it. From the console, after stopping gdm, enter this command:
ps ax | grep X | less
You'll get a display of all processes that have X in them. You should easily be able to identify the X-windows process (it is named X). Once you have identified it, kill it with the kill command.
Then install the NVidia driver using the NVidia installer.
Note that, to do this, you need to have the gcc toolset on your system and you need the proper kernel headers.
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