It sounds to me like you're reading two different things and getting them mixed up.
What's the FSB speed of your CPU? I'm pretty sure it's 333MHz, but some Athlon XPs do have the faster 400MHz speed.
Anyways, you have two different speeds. Your bus speeds, which should be the same speed as your memory is advertised at. Say you have DDR333, then your bus speed should be 333MHz.
The other speed is your Front Side Bus speed, which is determined by the CPU. If your memory speed matches your front side bus speed, it should be half that of the bus speed, or in this case, 333/2 = 166MHz.
Download CPU-Z, this will provide all this information and allow you to post screenshots of it so we can check it out.