So you installed your Linux server with gnome or kde and now want to boot to text mode, mostly to save some resources that are being used by your X server…

Debian:
go to /etc/init.d and execute:
update-rc.d -f gdm remove
for kde:
update-rc.d -f kdm remove
to restore:
update-rc.d -f gdm/kdm defaults

The above command works for pretty much any script you want loaded during boot. Simply copy your script to /etc/init.d or make a symbolic link and execute the command.

CentOS/REHL:
Edit your GRUB
nano /boot/grub/menu.lst
and add runlevel you need to the boot line. Text mode is usually runlevel 3:
kernel /vmlinuz-2.6.9-78.0.1.EL ro root=LABEL=/ rhgb quiet 3

SuSE:
Edit grub and add level 3 to your boot line:
kernel /vmlinuz-2.6.27.21-0.1-pae root=/dev/sda5 resume=/dev/sda9 splash=silent crashkernel=128M-:64M@16M showopts vga=0x317 level 3
You can always run to graphical mode by using the command:
startx

Other distros: google it 😛