You need reconfigure monitor V and H sync range. You can find out correct sync value by visiting Samsung Syncmaster 753S website or by reading manual.
Once you found out correct values, boot RHEL into single user mode or If you cannot start the X Window System and you have only a black screen or bad looking screen, press [CTL]+[ALT]+[F1] to get to command line. Login as root and type the following command to reconfigure X including Video card, Monitor etc:
Code:
init 3
redhat-config-xfree86 --reconfig
Note: If in doubt, leave the settings at their defaults when you don't know what to supply for settings...