The first and BIGGEST difference insulating can make is 1) Stop the flow of air.
Enclosing the underbelly in a fairly airtight manner is the first and most important step. If air flows through it, the insulation is of much less benefit (especially fiberglass, which relies on trapped air for insulating)
Air is a pretty good insulation, if you can trap it. Just remember to think about moisture in trapped air spaces. Another benefit to furnace heated basements, the airflow helps remove the moisture (can add it too, not perfect)
If you use XPS foam and seal it tight with little or no air spaces, less opportunity for moisture to form and creates a good thermal break.
Good luck ;)
