There are good references to disabling the nouveau driver in the docs I pointed out to you, but I give you the quick lowdown.
1) Install debian nvidia pkgs using your favorite method (aptitude, synaptic, apt-get...)
these 3 packages should pull in all the required dependancies:
2) blacklist the nouveau module
edit /etc/modprobe.d/blacklist.conf (create it if it doesn't exist) and add these lines
options nouveau modeset=0
The first line prevents the module from being loaded at boot; second line prevents the nouveau module from doing a kernel modeset if it somehow gets loaded otherwise.
3) This may be unnecessary, but I do it for completeness, in case the nouveau module might get loaded during boot by the initramfs:
~# sudo update-initramfs -u -k all
~# sudo update-grub
This, of course, assumes you use grub to boot, so if you use lilo or something else, you will have to make sure it is properly updated as to the location of the initramfs file.
This is often hard coded by the boot loader.
3) Reboot and even if something tries to load the nouveau module, the 'modeset=0' line will enable you to properly unload the module.
Now I find it best to go into single-user mode. You can accomplish this by either booting from grub into rescue-mode or from command line with
Now the environment is clean and uncluttered to setup the X config file.