Better living through higher voltage
That's right, after you've hit a wall, the next step in your overclocking odyssey probably involves voltage.

Our example rig gets a minor bump in CPU voltage

Increasing the voltage applied to a given system component usually helps it run at higher speeds. This is particularly true of CPUs, but be careful not to get carried away. Raising processor voltages will make a chip run hotter, requiring additional cooling. More voltage isn't always better, either. Unless you're using extreme liquid or sub-zero cooling, most chips tend not to benefit from more than a couple of extra tenths of a volt. Start out with small voltage increase increments and initially confine your voltage fiddling to the processor. Once you've applied a little extra voltage through the BIOS, shoot for the last front-side bus speed that failed to see if it's stable. If system stability gets worse when higher voltages are applied, it's time to beef up your cooling or back off to lower voltages.

When you reach a point where increasing the CPU voltage doesn't result in higher stable clock speeds, turn your attention to chipset and front-side bus voltages. These can be increased, as well, and they tend to be most helpful at higher front-side bus speeds where you're approaching the limits of the motherboard's capabilities. In my experience, you'll run out of CPU headroom long before your motherboard gives up, but it's worth tweaking the mobo a bit just in case.

For the record, our example CPU eventually turned out to be quite happy at an astounding 3.64GHz at 1.3875V, using nothing more than a stock Intel air cooler. We stepped through at least 10 to 15 different clock speed and voltage increments in order to reach this speed, recording info as we went.

Holy Moses!

Be sure to document all the settings you used, your system temperatures, and the results of your stability testing along the way. This information will come in handy when you've determined the limits of your processor and motherboard. For some, the point of overclocking is to achieve the highest speed possible, at any cost. However, for others, it's about reaching an optimal clock speed with the best blend of performance, power consumption, and temperature—the sweet spot, if you will. This sweet spot may give you 90% of the highest stable clock speed your processor can achieve, but do so without the need to increase voltages or significantly impact temperatures. I'll take that over bragging rights any day.

When you settle on a final configuration for your system, you should conduct a longer stability test, otherwise known as a burn-in. This test should peg your system at full utilization for several hours at the very least to ensure that everything is perfectly stable. It's a good idea to log temperatures during this test to ensure that prolonged periods of heavy load don't overwhelm your system's cooling solution.