Re: Switching from ATI to NVidia
edit your XF86Config in /etc/X11
remove anything it is ati specific (mostly in the device section).
Then try to run (after entering the init 3 mode) the nvidia installer. Make sure there is "nvidia" in the section "device". It will certainly work. At least it did for me. Btw I don't think you actually need to reformat your disk.
PS: read the README file that comes with the driver for available options.