As part of a recent upgrade to my computer I purchased an Nvidia GeForce 8600GT. While reading about it on the Ubuntu Forums I noticed that a lot of people were having difficulty getting it to work in Ubuntu. I researched the Ubuntu Forums for the answer and eventually found it, but I thought I would post the simple version here.
To start with, I was installing Ubuntu from scratch because I wanted to upgrade to the 64-bit version to match my new processor. Unfortunately the live CD was unable to correctly configure my graphics card. It was using an incorrect video mode that was making my monitor freak out. All it showed was a black screen, but when I reset my computer it still gave me a black screen (when the BIOS screens should have been visible). Powering it off and on didn't help, so I had to actually unplug my monitor's power and plug it back in to get it to stop wigging out.
In order to install Ubuntu I had to download the alternate install CD. Obviously my graphics card was able to handle the text-mode installer, and after booting Ubuntu for the first time, again to a black screen, I hit CTRL-ALT-F1 and edited /etc/X11/xorg.conf to change the "nvidia" driver to the "nv" driver.
This is only a temporary solution because the "nv" driver is not 3D accelerated, so the final step is to download and install Envy and allow it to install the Nvidia driver for you. Envy recognizes the 8600GT card and correctly configures your xorg.conf.