PC800 45ns RDRAM is made for P4 400MHz FSB processors. You can use PC800 40ns RDRAM on 533MHz FSB P4s, but they'll most likely be running at PC1066 speed or they may default to a safe PC800 speed (400MHz FSB). But the minimum set speed for that ram is probably set at PC800 40ns (if it is a Dell machine). If you want to be safe, just get PC1066 16bit memory. 😉