do u have a console login set or a graphical one?
if ur settings are wrong, then ur graphical login should fail, and take you to a console login screen. alternately, you can always do a "ctrl-alt-f1/2/3/4/5/6", and be taken to a cmd-line.
u have to edit ur /etc/X11/XF86Config-4(or something of the sort) file.
i think you can do this in redhat using "redhat-config-xfree86' or something of the sort.