newer P4 processors have whats called 800mhz Front Side Bus, it forget what it actually is, but i do remember that the speed of the processor depends on it. older P4 processors have a 533mhz Front Side Bus. Hence a new processor (one with an 800mhz FSB) wont work with this board. when you boot up your computer one of the first things that it will say is either "533mhz FSB" or "800mhz FSB" (im pretty sure i never pay attention to the POST screen anymore...)