Have you checked the monitor sync rates also?
Those are usually set in your /etc/X11/xorg.conf where it says:
HorizSync 30.0 - 83.0
VertRefresh 50.0 - 76.0End Section
I also assume you have restarted your X server after making all these changes by pressing ctrl-alt-del or rebooting?
NOTE: Those refresh rates are taking from my 1280x800 WXGA lcd monitor. I wouldn't recommend you using those on yours. I would check your manufacturer's website for those specs...