your motherboard is what determines what memory FSB is supported. CPU and memory FSB run independatly of each other. your CPU could run at 333Mhz FSB while the memory runs at a 400 or greater FSB. its is good to run them in sync, but not neccesary.
you can easily increase the xp2500 effective FSB from 333Mhz to 400Mhz. you could even overclock your CPU past 400Mhz effective FSB. same as with memory, you could probably use pc3700 with a motherboard that supports pc3200 maximum.