For Redhat, you can use Xconfigurator to set up your resolutions. Also, see Aussie's post in this thread
concerning checking/editing your resolutions manually in /etc/X11/XF86Config-4.
Once you set multiple resolutions in the config file, you can change your res on-the-fly in Gnome, KDE, etc. by simultaneously holding down CTRL+ALT+the numeric keypad's "+" or "-" key.