sudo apt-get install ubuntu-desktop
and the dependencies:
apt-get install x-window-system-core xserver-xorg gnome-desktop-environment
sudo apt-get install ubuntu-desktop
sudo apt-get install gdm
sudo /etc/init.d/gdm start
sudo dpkg-reconfigure xserver-xorg
Gdm is what handles your x system starting automatically instead of entering start x at the command line each time you boot. The reconfigure runs the xserver setup so you can configure your system monitor, video card etc.
