It is possible to install the nvidia drivers while running X. (And come on, this is not Windows - anything can be done any time .)
If you are using the installer from the website, running the following would have given you what you wanted:
. My other favorite options are "-K", i.e., "--kernel-module-only" and "-k", i.e., "--kernel-version".
sh NVIDIA-Linux-x86_64-195.36.03-pkg2.run --help
sh NVIDIA-Linux-x86_64-195.36.03-pkg2.run --advanced-options
sh NVIDIA-Linux-x86_64-195.36.03-pkg2.run --no-x-check
Make sure to add the line
in Device section of your xorg.conf and then,
should restart X with green drivers.
Btw, I am on Gentoo and I just use the distro package ("emerge nvidia-drivers") instead of the installer. Mainly because it has a rather awesome and sophisticated way of linking the opengl libraries. If Debian and/or Fedora or any distro ever get the equivalent of "eselect opengl", let me know .