Breaking Dawn would best be done in two movies. There's just too much detailed story to cover. If it's only done in one movie, Summit might as well call in a butcher to be the director because it will be hacked to death.
For continuity sake, because the first three movies are not in 3D, I would prefer Breaking Dawn also not in 3D. If it is done in 3D, James Cameron (Avatar) should be the director, and I doubt that will happen.