FSB overclocking is just fine, but a higher FSB means higher memory bandwidth, so overclockers who have multiplier-adjustable chips go for the highest FSB possible, then drop the multiplier to reflect the maximum clock speed of the CPU.
For example, say my A64 2ghz has a default FSB 800, multiplier 2.5
I find that I can overclock the chip to 2.4GHz.
Rather than running 800 with a multiplier of 3, I raise the FSB to 1200MHz, then drop the multiplier to 2.
If I didn't drop the multiplier, I wouldn't be able to have such a high FSB. see?