Overclocking by the FSB s more dangerous because you are boosting the operating frquencies of the PCI, AGP bus etc.
And so your stressing your system more, and pushing the various cards beyond operating specifications. However it can also yield more of a performance boost than overclocking by the multiplier because with overclocking by multiplier you are only getting a higher clockspeed. Overclocking by FSB boosts many more variables.