As far as I remember, my P4B533-E (before I burned it due to an unsuccesful Vmem mod) after applying the dip switch trick, had to set 1:1 in bios in order to clock the mem @fsb x1.333(2.666 ddr). If I chose the 3:4 divider, then the memory was running @ a true 1:1.
So, give it a try and post any...