"It is hard to draw any concrete conclusions from this amount of limited experience but one thing is glaringly obvious. All Bartons are not created equal. Some simply are not up to running at 200MHz FSB even when every other component on your mainboard is up to the task. Looks as if the luck of the draw has a lot to do with it.
Also, we tried underclocking the CPUs as well while still running an overclocked FSB, but this made no difference in the stability of the 2500+s or the 3000+.
Now the next question that you might be thinking is what do I really loose if I am "stuck" at 166MHz FSB. We thought we would look at this as a worst-case scenario as we have not seen any chips that would not do at least 185MHz FSB stable."
More here.
Update by Per: You might be interested in taking a look at this site and their findings regarding the product code all AMD CPU's has... Disappointingly though [H] did not include the product codes of the CPU's they tested.