Simply adapt HozizSync And VertRefresh to values matching your new Monitor, see section below, you find this in /etc/X11/ in the XF86Config file.
CAUTION!!! DO ***NOT*** OPERATE YOUR OLD MONITOR with the new settings if those values are higher, or you might destroy it!
Run X again first when you attached the other monitor!
Section "Monitor"
Identifier "107035"
HorizSync 30-95
VertRefresh 50-120
EndSection
> I would like to change monitors for my box. How do I do
> this?
