U can also Try "Redhat-configure-display" at the command prompt.
About my early answer
become the root
1,type "XFree86 -configure" at the command prompt
2,A file called Xfree86.conf or something will appear in ur home directory
3,move that file to /etc/X11 directory after removing the *.conf extension using "mv"
restart the xserver.
any of these two will solve ur problem
(i dont remember the actual file names as i dont have access of linux for a long time)
also as said earlier plz inform me if worked