with a FSB clock of 133mhz and a CPU of 2.8Ghz it uses a muliplyer to get from 133 to 2.8 in this case 21X133 = 2.8 which is the processor speed. The processor system bus of 533mhz is (I think) how fast the processor can calculate things internally, so a 533 processor bus will be quicker (in theory) than a 400mhz processor bus.