Is the FSB for your Athlon XP 1600 also 200 though?
If so you should set the mobo to 100 FSB, the Athlon takes care of the doubling. All Athlon XPs are named with the perceived performance level, not their actual speed. Therefore yours will not measure at 1600 but at the lower physical figure, although I'm not sure offhand what that is for your XP 1600.
Go to the AMD website and it will tell you that figure though.