Cooling

Cooling is definitely one of the most vital components of a good overclock. As power draw increases, to put it bluntly, stuff gets HOT! Fail to keep the components within the operating ranges, will result in either damage, or death of that component very quickly, especially at high voltages. As a general rule, I take the manufacturers specified thermal ceiling, and for every increase in voltage, lower the ceiling. I've concluded this by simply observing a few basic facts:

1. Mobile Athlon XP's are rated 100C maximum
2. Desktop Athlon XP's are rated 80C maximum
3. Mobile and desktop Athlon XP's are as far as we know, Identical, just speed binned for better speeds.
   

While I have no proof of this rule of thumb, I have yet to result in a dead CPU or GPU because of heat and/or voltage.

Increasing the speed increases heat output, but when you increase the voltage, it amplifies the heat output by much more.

A rough example of how increasing the voltage will effect the heat output/power usage on an Athlon XP. As you can see, MHz itself doesn't have as much of an impact on power consumption, although, large jumps will increase the angle of the line by a significant amount.

Types of cooling

1. None - Not a good option for almost anything out- especially overclocking!
2. Passive heatsink - More common with older systems, such as Pentium’s, Pentium II's and Pentium III's, as they generally can handle higher temperatures and still run fine, along with the fact that their thermal envelope is very, very low (in range with most low power laptops today), so a large modern heatsink, such as an XP-120 would keep it cool enough to not even need a fan.
3. Active air cooling - Probably the most common form of cooling today, relies on fans to amplify the amount of airflow over the heatsink and cools much more efficiently, but can be extremely loud, and has its physical limits, such as size of the heatsink, maximum airflow through the fins on the heatsink, and the ambient air temperatures.
4. Water cooling - A basic loop of water, running over heatsinks designed for this purpose, known as waterblocks. Much like active air-cooling, but instead of air, water is used. Temperatures using this method are generally lower than most air cooling, and a good water setup should be able to outperform the highest end air cooling setups, and quieter, to boot! The main components are the waterblocks (heatsinks), the tubing, pump, a radiator to dissipate the excess heat into the air, and sometimes a reservoir.
5. Thermoelectric cooling (TEC, aka peltiers) - refers to a "heat pump" type layer that is sandwiched between the CPU and another cooler (air, water or another TEC). It generally needs double the wattage of the heating element - in this case, the CPU. Has a hot side and cold side depending on polarity (cold side on the CPU!) and end up increasing the internal temperature of the case substantially, unless the TEC is cooled externally. The advantage is it's noiseless. The cold side is placed against the CPU, and warm side is cooled by another means - water or better for a good setup - since it puts out it's own heat, plus heat from the CPU. (~Beomagi) While generally safer and easier to utilize rather than building your own phase change, it isn't without its dangers! There are many horror stories of people leaving peltiers on, or their water pump dieing, and the peltiers then burning up not only the waterblock, but the CPU and motherboard as well!
6. Water chillers - Using the evaporator of a phase change system to cool a reservoir of water. This setup is simpler and safer to make for beginners, as it can be done as simply as taking apart a window air conditioner, slightly bending the evaporator, and building a reservoir around it. Turn it on, (make sure you take the fan out!!) and within minutes, you have chilled water! General expected water temperatures: -15C
7. Phase change - Phase change is similar to water, as it is a chemical in a closed loop, flowing through a block, but a little more extreme. Instead of water flowing over the block, it uses a gas at a high pressure to create a liquid. As the liquid flows into the CPU or GPU evaporator, it evaporates and in doing so, basically "sucks" the heat out of whatever it can, to gain the needed energy to turn back into a gas at the pressure. The evaporator line is insulated as much as possible to keep it the coldest where it will meet the CPU. As the gas returns, it created a high pressure, but this time as a gas form, and needs to be cooled. The condenser cools this part of the cycle. The compressor controls the pressures. General expected evaporator temperatures: -40C
8. Cascades - A cascade, is basically a dual phase change system. It uses what you could call "normal" phase change system to super cool the condenser of the second stage. This allows the use of much colder gasses in the second stage, which are unusable in a normal single stage system. For a 3-stage system, it is just repeated once more. This type of cooling isn't suited for 24/7 usages, as running 2 or 3 compressors uses an extremely large amount of electricity and is primarily for long benching runs. Temperatures of a 2/3-stage system can approach and exceed -110C.
   
   Autocascades are a new and complex design utilizing a single compressor system, and using two gasses, separating them to act as a normal 2-stage cascade. This creates more strain on the compressor, but allows for much colder temperatures than what you could achieve using a single stage system. It works by just separating the gas and using it to cool the condenser of the first gas, rather than using a completely separate loop for that gas.
9. Dry ice/liquid nitrogen - Unlike the other methods of cooling this is the least permanent. Using dry ice or liquid Nitrogen requires a "container". It is an open-ended tube with copper at the end, which is on the CPU or GPU. Using dry ice, acetone is added to the container to increase surface area and lower overall temps and increase capacity. Liquid nitrogen is a liquid, so no additives are necessary. Both methods sublimate/evaporate very quickly, so refilling often is necessary. General temperatures are -60/-160C respectively.
   

Ultimately, your cooling choice is up to you. Look at what your goal is with overclocking, and pick whichever best suits you (Along with your budget!). For a light overclock, and learning, it’s best to start with an upgraded heatsink. For the more experienced users, water-cooling is a very good option, and for those looking for excellent operating temperatures, phase change is a good way to go, BUT those wonderful temperatures come at a price!