Depends on the system, if it's an AMD Socket A system then going above 400FSB means you're overclocking the whole mobo's system speeds which means you'd generally need more voltage to the north bridge which means more heat.
Read into overclocking more and all these kinds of questions will be answered.
Just realize on an AMD64 they are all stock 200HTT....you CAN go above that like 250HTT and then that would match with DDR500 RAM, BUT you'd need to lower the link speed multiplier on the AMD64 so that your total board bandwidth stays at 2000HTT....but from the sounds of it you're more talking about a socket A system which I just explained above......Yes it's possible, but to go beyond that on a socket A system is harder than on the AMD64's