its kinda confusing the way they market the chips, but... a "200" athlon/duron is designed for a 100mhz fsb (the bus on athlon/duron boards is double pumped so they call it 200, but technically its only 100mhz)
so your xp is designed to go on a 133mhz fsb, not a 266mhz
but to answer your question.. the reason you dont have a setting for 266mhz fsb is cause your board isnt capable of even getting close to that speed