I wrote about vitamin D a few weeks ago, and I recently read an article that helped me pin down some safe dosages.
In an article in the Washington Examiner, Dr. Oz rejects mega-doses (40,000 IU/day), indicating that women who took them were actually more likely to have bone fractures. He says that 1,000 - 2,000 IU/day is probably OK to take without prior approval from your doctor. He says 4,000 IU/day is probably the upper limit, and that you should talk to your doctor about it first. Since my levels are clinically normal but on the low end of the spectrum, I'm not terribly worried but I do plan to ask my doctor what he thinks at my appointment in a month and a half.