Voltage is a messure of electrical force. Amperage is a measure of volume. And you also have resistance.
Imagine a water wheel with scooped paddels. Each paddel of the wheel represents a chunk of data. Now imagine the water line has a filter. This filter represents resistance. Now imagine a gate that opens and closes to let the water through into those little paddels. This gate represents your clock generator.
OK, now, you set the gate to open and close a certain amount of times per second. Everything works good. So you keep turning it up little by little, unit it's finally opening and closing so fast that the paddel buckets no longer have time to fill...you can't make the wheel go any faster...unless you increase the volume of watter through the pipe. The only way to get more volume through the pipe is to increase pressure through the filter. You can't simply increase the size of the pipe, you need pressure.
OK, well, another problem is that you will eventually reach a point where the wheel can't spin any faster, because water wheels are gravity powered and gravity has a finite rate of acceleration, that's your upper limit.
In the CPU, increasing voltage allows you to push data at a faster rate through the resistance of the circuit. But it also increases the amount of heat. And heat makes the CPU unstable. Adding even more voltage helps overcome that instability, but creates even more heat...you've reached a limit.
Generally the best voltage to use is something less than that point where all the extra voltage is lost to heat. Some processors will burn out before they reach that point, most won't. But many will burn out from a hot spot, over time. I'm not going to talk about electro migration and so forth because it's not pertinent to such a basic discussion.
Anyway, you'll find that there's a certain point where adding more voltage no longer helps you. "The wheel just can't be spun any faster".
The best way to find the limit is experimentation. Experimentation shows me that with air cooling, voltages above 2.8v don't help the K6-2. Voltages above 2.05v don't help the PIII Coppermine. Etc. Each processor varies a bit, but you can come up with some fairly close approximations.
Voltages above 2.4v didn't make my K6-III run any faster. Based on my knowledge of the K6-2, this seemed reasonable, so I recommend 2.4v or less on an overclocked K6-III.
ATX power supplies often help AGP equiped AT boards run their AGP cards with more stability, because many can take their 3.3v input directly from the power supply, instead of using the weak onboard 3.3v regulator (AT power supplies have no 3.3v line).
<font color=blue>Watts mean squat if you don't have quality!</font color=blue>