You can prevent automatic running of the GUI when you boot your debian machine by disabling your login manager be it KDM, GDM or XDM from running at boot time. To disable the login manager from automatically running at boot up, run the following command as root
#update-rc.d -f gdm remove
Replace gdm with kdm or xdm if they are what you use.
To start X manually, you would then have to login at the command prompt and enter the command startx.
To reset your login manager so that it runs at boot up, do
#update-rc.d -f gdm defaults