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:
Code:
telinit 3
redhat-config-xf86
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"...