it will probably run but at a lower clock speed, the clock speed of a CPU is normally worked out in the following way
FSB * Multiplier = CPU Clock
so if you plug a 533 processor in to a 400 FSB it will run slower.
Unfortunately the Multiplier is normally locked by manufacturers to stop overclocking.
My advice would be to buy a new cheap motherboard (get an Asus there only about £30)