1) How do you determine which GUI is started when you start the computer? By default my RH8.0 installation is starting GNOME when it boots. Is there a configuration file where this can be changed?

2) Is there any way to kill the GUI and drop to a shell (rather than running a shell in a window) ?

Usually its going to be specified in your user's home directory, either your .xinitrc or I think Redhat might even specify it in your .Xsession file.

If your starting X by actually typing startx, you can kill it with a ctrl-alt-backspace
If your booting to X directly, you can maybe ctrl-alt-F2 to get to a terminal and kill X that way by finding its process id to kill.
Or at boot time you can specify to boot directly to runlevel 3 instead of 5.
You can also edit your /etc/inittab file to change the default runlevel to 3 instead of 5 as well to never boot straight to X.

Normally there's no need to "kill" the GUI, just press ctrl-alt-f1, and then ctrl-alt-f7 to get back again

To change your default level, look in /etc/inittab and change your default run level from 5 to 3.

You must make sure you do not change it to 6 or 0 or you're screwed and no one here will ever help you again.