Well you can either run xf86config and when it prompts you for monitor settings just tell it what you want
goto /etc/X11/XF86Config edit the file manually at the Monitor section. Just be carefull not to set the frequency ranges past what your monitor can handle. Your Distro may also have an easy gui utility to do this.
One more thing as advice before either editing or running xf86config rename XF86Config to XF86Configbk just in case