The key to insulating the rafter cavity with fiberglass (or rigid foam insul) is to fill the cavity completely with insulation, without compressing it. You do want the air pockets to be maintained within the insulation. If you fill the cavity completely, then ventilation is not required at the eaves. The key to stopping the moisture is to have a continuous vapor barrier on the warm side to stop the warm moist air from getting into the wall and condensating. Condensation occurs as soon as the moisture in the warm air travels through the wall and reaches the dewpoint, which is approximately half way through the wall.
If you don't completely fill the cavity with insul then you will need ventilation at the eaves and the vapor barrier.
If your using the ridgid foam insulation the sheets already have a vapor barrier so taping the joints is definately needed. Be careful with the foam types of insulation because most of them are required to be cover with sheetrock because they are highly flammable. Some types are listed for use without being covered with sheetrock just check the labels on the sheets. Also I think fiberglass is a lot cheaper.
Hope this helps...
