the 533fsb would most likely get you to the higher FSB region meaning by clock to clock comparison the 533fsb with lower multiplier would give you an overall faster system.
hence the 533fsb has a 18x multiplier <--- seems to overclock higher too...range of 2.8-3.0...with a new C1 stepping core, you can most likely achieve the big 3.0GHz+
and the 400fsb has a 24x multiplier