I haven't tried it on Ubuntu yet, but you should be able to set up the OS to start at runlevel
3 (rather than runlevel 5) so it will start with no GUI.
You can also keep the GUI (any combination of Gnome, KDE, LXDE, etc) installed and you can switch to GUI mode by using
You can also uninstall the GUI later, and if your computer is set to runlevel 3 you'll have no problems.
If you don't want to go through all that, you can also just leave your GUI at the log-in screen (or any screen you want for that matter) and press CTRL+ALT+F[1-7] to switch to various terminals to log on for terminal only.