There is more to ISO than noise. There is loss of details and loss of dynamic range. D200 is best at ISO 100, I use it much higher if I have too. How far up you will go before some of the high ISO issues become too severe, depends on the picture and your expectations.
As you allready have the camera, I will advise you to do some tests yourself. That will probably teach you more about your camera than what you can read on the net.
Thank you for the reply, i was wondering if the D200 has any ability with ettr and higher Iso speeds that would sucsessfuly lower the noise, like being able to use a speed of 400-800Iso and through ettr and ec, actualy achieve an equaly clean file sutch as 100-200Iso, i know this is quite common and used susessfuly with canon cameras.