the time when I bought the parts to build, only 64 bit P4 CPUs were available, and I was told the price was just a few AUD more, so I took the the 531 EMT 64. fyi, my XP SP2 is only 32 bit, so did not make much use of the 64 bit function.
I knew it only use DDR not DDR2 but did not know it does not support Dual Core CPU, but anyway, as it was not expensive ( only 80+ AUD ), so I did not expect to upgrade it ( only upgrade the BIOS ), I would get a new Mobo if a faster CPU is needed in the future.