What FSB options does your motherboard have? If you're bumping in low increments, default voltage will usually work (my experience, anyway). But when it gets to larger jumps, more voltage is need (little bumps here, too).
Everything else does actually run out of spec as the FSB is puched out of spec. It's all based on the FSB frequency. There are dividers which control what speed everything else runs relative to the FSB. At 66 MHz, the PCI divider is 1/2. The AGP bus runs at 2x the PCI bus. So at 66, the PCI runs at 33 and the AGP runs at 66.
Most contemporary boards have dividers that run to 1/4 (though some older ones stop at 1/2 or 1/3). The next divider line is usually 84MHz (half way between 66 and 100). So a 100 MHz front side bus yields (per spec) a 33 PCI and a 66 AGP. The next divider usually kicks in at 124 (1/4). You're smart enough to see what happens at the next spec level of 133.
The problem comes in when the FSB is raised. The hardware was designed to operate at certain speeds. Anything outside of that can cause problems. Especially with poor components.
A good analogy might be the tires on your car. Tires are speed rated, too. This doesn't really concern most of us since we (unknowingly) buy tires whose rating exceeds the speed we drive. But imagine that Mr Indy Car Driver (read Overclocker) takes the tires off your Pinto or minivan or whatever and puts them on his nice little machine (work with me here
). His car will drive along relatively well at the speeds you and I see in the city. Even highway speeds wouldn't be a problem.
But what happens when he hits the back straight and punches it to 200? Bad ****, that's what. Tires pieces start flying everywhere. Our tires were speed rated for only 100 MPH. They stayed together for a (very) little while running over spec. But when Mr Driver puched it too far, they were gone.
If he had driven at say 120 or 130 or some number that's still too high but not WAY too high, then he would've experienced instability. The tires heated up from being pushed out of spec. Maybe a couple of them started developing heat bumps or thorwing off little pieces of tread. That makes for one helluva poor ride.
Hope that little story helps with understanding the spec issue (though it was far too long of a story).
If you're limited in FSB choices on your motherboard, you may be out of luck unless you're willing to risk the processor by guessing at high voltages. That's what I had to do with the Celeron 400 in my sig. I had too FSB options: 66 and 100. Set it to 100 and set the slotket voltage high. It was just a toy though. Nothing too important to break.
