By default Fedora 10/11 does not have /etc/X11/xorg.conf then you xorg can not configure monitor to the right Refresh rate. Under Gnome/KDE I can not find any configuration tools to change this. To do this you have to type several commands and edit something on xorg.conf
To generate the xorg.conf based on your hardware:
su -c ‘Xorg :1 -configure’
su -c ‘vi /etc/X11/xorg.conf’
Then add your HorizSync and VertRefresh value under Monitor section.