this seems to happen 10 times a day. if you go above 133, you need to make sure that your components can handle the higher pci and agp speeds. also, don't try to just flick the chip to 166. you'll need to lower the multiplier. i would try this prgression: up the mutliplier one half step at a time until you have problems. when you hit this speed, put the multiplier at default and then try to up the FSB until you get to the same speed you were at when you were upping the multiplier. if you can't get the FSB overclocking to be as high as the multiplier, then screw around with various combos or FSB and multiplier until you have the best perfomance at a stability level you are comfortable with. as long as you have decent cooling the very worst that will happen is that you have to clear the CMOS (using the jumper by the battery) and start the process over again. maybe you'll be lucky and run the system at the 166 FSB (though i doubt it, unless you have top notch stuff across the board).
good luck!
--jacob