ive done this before (it looks very pretty when you do it
basically what you are doing is using incompatible refresh rates and resolutions. monitors can only sustain certain resolutions running at certain refresh rates (they'll be noted in the manual).
you have to be careful, you can fry the monitor very easily doing this (although things are better than they used to be, XFree86 version 4 tends to kill the X server is it detects that the monitor/graphics card cant handle the settings. also monitors tend to have large ranges of refresh rates so it is more difficult to break them.
look in the manual for your monitor to see what rates and resolutions your monitor can handle then configure XFree86 with this information