Stopping the bad boy
Yep, it's simple. Linux usually has virtual terminals available.
To get to a command line terminal use ctrl-alt-F1 (F2-5 also work).
Change the run level to level 3 from level 5 by issuing the command telinit 3 . This puts you in full network and multiuser mode without X.
Install the Nvidia driver.
Then switch back to run level 5 with the command telinit 5.
Suggestion: make your edits to XF86Config before you do the install. It's easier in a text editor in X than in command line mode. Also, make a backup of the file PRIOR to the edit (copy it to say XF86Config_7_9_2003_backup). May save you some hassle if you have problems.
BTW, you can switch back to X (if running) from a virtual termal by using ctrl-alt-F7 (F7 normally but could be on F8). This shouldn't be needed for this exercise but it's handy to know.
Hope that this answers your question on killing X. I leave the Nvidia driver config and install to you and their instructions.