Magnesium is an essential nutrient - but most of us get enough, so there's really no need to take a supplement. I've seen no scientific evidence that taking magnesium with calcium makes a difference for bone density. There is some interaction between magnesium and calcium, but the effect is not enough to cause problems when calcium consumption is in the normal range. If you decide to take magnesium, don't go above 100% of the RDA, which is 280 mg.