Vit D levels are easily tested for. Determining your needed dose should be done based on that, from your doctor.

However, the RX form of D = ergocalciferol D2, and is not really active in humans, but still given in doses of 50,000IU weekly.

D3 OTC is the form to use (cholecalciferol), but many doctors remain uneducated about this difference.

The recommended dosing scale is 1000IU D3 for every 10 pts to raise to the desired level. 50 is often used, but people with MS are told often by their doctors to go higher (up to 100).
This is highly controversial and and should only be done with a doctor's supervision.

Many people on these boards end up testing below 10. Those individuals often end up using 5000IU D3 daily.

Here is our D thread:
http://neurotalk.psychcentral.com/thread92116.html

Taking high levels of D without testing or "just because" can lead to hypercalcemia. So getting the easy test is really important. It is also recommended now to lower any calcium supplements to 600mg a day or below if you decide to take Vit D supplements. Vit D increases absorption of D from foods quite a bit, so excess calcium is not needed anymore.