You can change this in the XF86Config file. iirc the file X on redhat uses is /etc/X11/XF86Config-4 (try locate if it isn't there).
search for something like:
Modes "1280x1024" "1024x768" "800x600" "640x480"
ViewPort 0 0
and change the Modes line to something your monitor can accomplish.
You can specify more than one mode as seen above, X will use the first one to start. Usually you can switch resoluction with <ctrl>+<alt>+KP_+ and <ctrl>+<alt>+KP_-
KP_* are the keys on the number block (is this the right term in english?.
 added the note.