I don't use redhat 9 but I know it uses gdm as it's display manager. Somewhere under the control panel, you should find a module named display manager or login. Something along those lines.
This is the same module that you can change the login text in.
You should find something named sessions with a dropdown box and a textbox to add fluxbox.
to make a lightweight kde...
Edit the "startkde" script. Near the bottom of the script is a call to "kwrapper ksmserver". Simply add "--windowmanager fluxbox" to the command, and KDE 3 will run fluxbox instead of KWin.
You may also just want to run fluxbox on its own, and run the "kicker" and/or "kdesktop" applications with it. This provides a sort of lightweight version of KDE 3.
to make a lightweight gnome...
I don't like gnome so I don't really play with the config scripts of it but I have tested this. Log into gnome and open up a xterm type "killall metacity" then type "fluxbox &" now.
Check out openbox also. I ran into it yesterday and am going to play with it when I get a chance. It looks nice: