First, you should check the manual for your monitor to find out the maximum horizontal refresh value. While on it, check the vertical too. It's important not to go over these values or you can be left with an unusable (black) display, or even fry a very old monitor.
Then find the HorizSync and VertRefresh values at /etc/X11/XF86config-4 and replace them from your manual. Do a startx and the best refresh rate should be automatically picked.