Start recovery mode.
dpkg-reconfigure -phigh xserver-xorg (go thru and if it asks choose "vesa" as a driver)
rmmod nvidia (remove nvidia module from kernel)
apt-get --purge remove nvidia-glx-legacy nvidia-glx nvidia-glx-new (purge nvidia-glx-legacy, nvidia-glx, or nvidia-glx-new)
rm /lib/linux-restricted-modules/.nvidia_new_installed (remove lrm cache that prevents installation of nvidia.com drivers)
edit the /etc/default/linux-restricted-modules or /etc/default/linux-restricted-modules-common configuration file and disable the NVIDIA linux-restricted kernel modules (nvidia, nvidia_legacy) via:
DISABLED_MODULES="nv nvidia_new"
(
http://forums.nvidia.com/index.php?showtopic=39818 )
Reboot to desktop. Download NVIDIA drivers to /home/$USER/nvidia.sh. Get NVIDIA drivers off nvidia.com that will support 8800 series. Latest in Feisty is 97.55. You may need 100.x+.
sudo /etc/init.d/gdm stop (stop gdm)
sudo apt-get install build-essential linux-headers-$(uname -r) (get gcc and linux headers required for module compilation)
sudo sh ~/nvidia.sh (install NVIDIA drivers, have it write you a new xorg.conf)
sudo /etc/init.d/gdm restart (restart gdm desktop)