The fastest Athlon XP uses a 200 Mhz bus that's double data rate, so it's 400 Mhz effectively. Intel's fastest is also a 200 Mhz bus, but it's quad data rate, so it's 800 Mhz effective. Theoretically, and in a perfect world, the 800 Mhz FSB P4's should have twice the memory bandwidth as an Athlon XP system... but alas, we do not live in a perfect world, and theoretical specifications aren't always indicative of real world performance.