well almost all of the 533a's ive tried do 800 fine at 1.65
a couple of the 566a's required 1.85volts and i hear 25% or more of the 566's dont even do 850.
If its not guaranteed to do 100fsb, you have a much better chance of getting 100fsb out of a 533 and then overclocking it more to its limits.
also use a MSI slotket only. the others will make it harder.