More on D from Dr. Dowd

"Vitamin D replacement is suggested in patients with levels less than 30 ng/mL with a target of 40-65 ng/mL as would be found in wild primates and in life guards and non-mechanized farmers who work in the sun. Nobody is suggesting replacing vitamin D in people with normal levels. Based on data compiled by Dr. Reinhold Vieth, there are no credible reports of toxicity in adults with levels under 200 ng/mL. The safety window for vitamin D is seven times greater than the threshold of normal. We are vitamin D deficient because we spend no time outside in the sun unprotected by clothing or sun screen, like our Paleolithic ancestors. Moreover, we do not eat the wild animal protein, wild seafood, and particularly organ meat that we evolved on, which was high in vitamin D.
"Vitamin D supplementation is required by most Americans because we are rarely exposed to sun and our diets are filled with grain-based carbohydrates that are devoid of nutrition. Moreover, vitamin D is stored in fat and only liberated with moderate or intense exercise, something that most Americans do not do. Our lifestyle is a perfect storm for vitamin D deficiency."
http://www.thevitamindcure.com/blog/?p=151
Keep in mind that vitamin D supplementation is VERY inexpensive, but only gel caps will provide benefit; tablets can work, but require that you take them with a fat source.  Also, most tablets seem to give a very small dose in a relatively large tablet, so they are much harder to take, especially for children.
How much do you need?  Enough to get your measured blood levels where you want them, and there's really no shortcut here - you have to get measurements if you want to know how much you need.  6000 iu gets me to the low end of the desired range - I'm going to remeasure at the end of winter, and adjust my intake next winter accordingly.