some other possible things todo
put the brand/type in your next reply,
maybe someone has the same and knows what to
put in your XF86config.
Does the monitor have an OSD? if yes does it tell
the refresh rate? Then you should try different resolutions
in windows and write them down.
Or do you have the monitor floppy or cd, then
check your inf file edit it and see what resolutions
are used with the freq range.
if it is a common 17 inch, a vert rate of 75 or 85 will
do fine and is most likely not putting it over the edge but then
again i could be so wrong