Stone_age was pointing out what often happens when you put an XP-M in a desktop motherboard. That combination usually defaults to a multiplier of 6, so if your FSB is 133 then your overall speed will be 800MHz (133x6).
To increase that you'll have to increase the multiplier. Hopefully you're right and your motherboard supports manual changes to the multiplier. If you can get it to 12.5 then you'd be at 133x12.5 or 1667MHz. If you can get it to 15, you'll be at the same performance of the XP2400+, about 2000MHz.
Of course increasing FSB also increases overall performance but it sounds like your system may not support more than 133MHz.
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