Try this: Let the computer boots until you reach the login screen and it turns black. Then press:
You will get into console login mode. Login as root and type:
The first command will kill your x-display manager, and the second( hope that command is right. It has been a while since I last used Redhat.) will re-configure your Xserver. You could, alternatively, go into /etc/X11 and edit the file XF86Config with your correct settings instead
When you are done, restart the display with "telinit 5". You can get back to the Xserver by pressing "ctrl + alt + F7"...