Calcium and vitamin D go together. So much of science is to do a study in which just one thing is changed and then evaluate the results. This is not good science, I my opinion, when there is more than one factor that interact in the body that could effect that study. IMHO, it is bad science to look at calcium without looking at vitamin D at the same time.
What we know for sure is that osteoporosis can be treated by taking vitamin D in doses recommended on this site along with calcium. In fact these studies suggest that vitamin D and calcium is superior to Boniva and the like. How much calcium? Well everything I read suggest less than 800 mg a day is all that is necessary and that limit should not be exceeded. How can this be true. Well, calcium just is not absorbed well unless there is a optimum amount of vitamin D in the body at the same time. So less is needed. In fact if you drink milk, eat any cheese and like your vegetables you probably do not need supplementation of calcium. Few people realize that vegetables contain some calcium also. In some societies no one drinks milk or milk products after weaning and they do quite nicely without any need for supplementation. It is also true that if the amount of protein in your diet is high, calcium is absorbed poorly. That society would be China, just in case you are curious.