The average person doesn't get nearly enough vitamin D, especially if you're housebound, or don't live in warm sunny climates. You want to shoot for as much as 70-80ng/ml; I would suggest 2000-3000mcgs daily. The current recommended RDA is WAY too low.
Just make sure to not bother with the synthetic version. And you also only want to take vitamin D with some type of lipid (fish, vegetable, olive etc.) As a precursor to cholesterol, vitamin D is only truly absorbed while administered with a fat soluble substance. Otherwise, you'd be wasting your time and money. Cod liver oil is also a great fat-soluble source of both vitamin D and A.