Just curious: why turn off gnome?
If you're going to use it as a gui-less server, install something meant to be run without a GUI. Ubuntu does NOT fit under that category. Possibly Damn Small Linux, or *shudder* FreeBSD with a Linux emulator. I personally use FreeBSD with a Linux emulator to run SourceDS on a low-spec computer.
press control+alt+F1 to get into a terminal.
login as root and run
Code:init 3
to get the gui back run
Code:init 5
as root.
you can also add it as a boot parameter in grub to make it boot into runlevel 3