Generally speaking, messing with the FSB or HTT speed on your processor is reversable. (I say generally because there'll have been SOME idiot who fried his processor doing it).
If anything goes wrong, you just reset your CMOS, and start again.
Voltmodding, however, is where the trouble lies. That's the more dangerous aspect, but vital for increasing the FSB in a decent way. You can normally ask around to find the range of voltages you could use. More Volts = Higher FSB, normally.
Really, though, you can normally get a fair increase just by increasing your FSB/HTT.