Try
redhat-config-xfree86
however, it usually gets to 'Trying default config' when it fails to load the old one, then fails itself. They really screwed up with the new tool. The best option. if you can, put the old monitor back on, bring it up, set to generic monitor, then reboot, swap it out, then change it to new monitor. The new tool doesn't even allow you to test, so be sure to record the backup in case the new settings don't stick.
Exactly similar to video card upgrade...hence this site I created, with this blurb...
http://wave.prohosting.com/hlrguy/Re...0/Xconfig.html
R.O.