What are the pros and cons of a vented vs unvented crawl space?
Most crawl spaces are vented if they have an earth floor or are prone to moisture. If you insulate between the floor joists with a moisture, mold and vermin resistant insulation (foam as we discussed before) you would still want some ventilation. The only time I would seal the exterior walls would be if I also used a pretty darn water tight moisture barrier over the floor, tuck taped to the knee wall at the bottom of the joists. You would be creating a dry cell and blocking movement of cold air through and under the house. This is typically done with a rubber/neoprene type compound sheet commercially, but can be done effectively  with a couple of layers of good 6 mil poly. This poly is avail at Lowe's, HD, etc. in 10 and 20 foot wide rolls, 25 to 100 foot in length. You can use Tyvek tape to seal the seams and around posts etc. Leave yourself a way to enter the space to check humidity levels occasionally for reassurance.