The FSB is the stock speed for the CPU to run at. If you go lower than that you're underclocking the CPU, unless you adjust the CPU's muliplier (which are locked nowadays) appropriately.

Here's the other thing: AMD bases their FSB on a DDR eqivalent when they put it up, and this does get confusing. The parallels are like this: 100=200, 133=266, 166=333, 200=400.

The problem you might run into is that the motherboard may not be equipped to deal with a newer CPU, like the XP2400+. It may not correctly recognize the CPU, even with a new BIOS. Sometimes the system will boot fine, but just tell you it can't reconize the CPU. In other cases, the system won't be able to boot at all. There's also the ability of a motherboard to supply the correct voltage to the CPU. As the CPU cores have progressed, using less and less voltage is one thing we see.