you will need to keep to 100mhz FSB
CPU's which should take you up to the heady heights of 850mhz with a p3 i think. im presuming your board is an Intel BX, if its a LX the previous version you would be stuck with 550mhz as the fastest. I recently made use of fleabay to buy an 850/256/100 p3 for a customer and it cost me a wallet busting £4.