Well you need to check you documentation that came with the monitor or from the makers website. Every monitor is different.
Here is a cut from my config to let you know what you need to look for.
VendorName "Monitor Vendor"
ModelName "PHILIPS 107T"
DisplaySize 310 230
HorizSync 30.0 - 71.0
VertRefresh 50.0 - 160.0
I think the things you'll need most are the HorizSync and VertRefresh. You may wanna check this out too: