Overclocking normally involves increasing the performance of the following components:

• CPU/motherboard (most common)
• Memory (common)
• Graphics card (least common)

Overclocking has traditionally been performed in the system's BIOS (show examples), but in recent years companies have started releasing software that will allow you to overclock components from within Windows. However, this type of overclocking is still pretty rare. Generally overclocking is done only on custom-built systems. While it is sometimes possible to overclock OEM systems like Dells and HPs, it is much more complicated than overclocking a custom system and will void the customer's OEM warranty, so it is very rare to see anyone trying to overclock an OEM system. However, there are companies like Alienware that build high-end systems using components that can be overclocked, and in some cases will build overclocked systems.

The most common goal of overclocking is to increase the CPU speed of the system. There are two parts to the overall speed of the CPU, the core clock frequency and the multiplier. CPU clock frequencies are expressed in MHz, exactly like memory clock frequency (such as 133MHz). Clock frequencies for recent CPUs range from 100MHz to 200MHz. The clock frequency is multiplied by the multiplier (which is locked on Pentium CPUs but usually adjustable on AMD CPUs) to give the overall speed of the CPU (i.e., 2.4GHz, 1.6GHz). Increasing the core clock speed of the CPU is the most effective way to increase the performance of the CPU, and it is the only way to overclock most Pentiums. Increasing the CPU speed, however, will cause the CPU to get hotter and can sometimes cause the CPU to overheat, potentially ruining both the CPU and the entire system.

Overclockers must compensate for this increased heat by using heavy-duty heatsink fans and thermal grease, which pull heat away from the CPU and disperse it into the case. Because the extra heat is dispersed into the case, the overclocker's case must have adequate ventilation. It is not uncommon for overclockers to have 4 or 5 case fans, which are positioned to move air quickly and efficiently into and out of the case. One negative side effect of all this increased cooling is that "air cooling", as using fans is called, can be extremely loud. Another option for overclockers is to use water cooling, which circulates chilled water against the CPU, lowering the heat silently. The main drawback to this setup is that it is much more expensive than air cooling, costing around $200, as opposed to around $50 for the most expensive air cooling system. Water cooling units are also relatively large and unwieldy, requiring much more space to accommodate the unit than air cooling systems.

Memory comes into the overclocking equation because most people want to run their memory at the same clock speed as their CPU, which gives the best performance results. This requires using high-end memory that can perform above and beyond its rated specs, much like an overclocked CPU. It is possible to run the memory at a lower clock speed than the CPU, but this will diminish overall performance of the system. Since most overclockers are high-end users who are willing to put a lot of money into their systems, most recognize the importance of having memory that will keep up with the CPU and prevent any bottlenecks in the system. This is why companies like Corsair, OCZ and Mushkin offer memory that runs well above the official JEDEC specification of PC3200 (DDR400). OCZ currently has the fastest memory available, at PC-4800 (DDR600). However, Corsair and OCZ have displaced each other's fastest memory on an almost monthly basis, so we expect this speed won't hold up very long, even though DDR1 is reaching the limits of its capabilities. Increasing the memory speed is a fairly simple process compared to CPU overclocking, since usually all a person needs to do is configure their BIOS to run the memory and CPU at the same clock speed and then increase the CPU speed, which will automatically increase the memory speed at the same time. On AMD-based systems it is possible to run the memory at a higher speed than the CPU, but generally the system performance is actually lower than if the two were run synchronously, so it is uncommon to see this done by experienced overclockers.

There are some other factors which play into memory overclocking, the most common of which is latency. There are 4 main memory BIOS settings that are collectively referred to as "latency timings" by the overclocking community. These are: CAS Latency, RAS to CAS Delay, RAS Precharge Delay, and Active to Precharge Delay, although each BIOS will refer to these settings with slightly different names. Each of these settings plays a role in memory performance, although the most important is the RAS to CAS Delay and the least important is the Active to Precharge Delay. A timing of 2 is the lowest possible for the CAS Latency, RAS to CAS Delay, and RAS Precharge timings. The lowest timing used for the Active to Precharge Delay setting is 5, but this setting is thought to possibly lead to hard drive corruption over time, so it is not recommended. The lowest recommended timing for this setting is 6. The lower the memory timings, the better the system performance will be. However, there are currently no modules available that can maintain these timings above PC3200. At least one, but usually more, of the timings need to be raised to accommodate the increased clock speed. The first setting that needs to be raised is the RAS to CAS Delay, as the write operation this setting performs is the most difficult and therefore needs more time to recover between write operations at higher speeds. For example, at the highest current speed of DDR600 (a memory clock of 300MHz), OCZ uses the timings 3-4-4-8, which are the highest possible timings to give the memory as much time as possible to recover at such high speeds.

The performance benefits of overclocking are hard to measure effectively, because one of the only ways to measure performance difference is through benchmarking programs. These are software programs that measure differences in things like calculations performed per second and frames of animation drawn per second. There are a couple of problems with benchmarking as a performance gauge. First, these programs simulate artificial system use rather than gauging "real world performance", so the results you get from them may not be indicative of true performance benefits. They also tend to only focus on certain types of results, so they may show better performance in games but not in "normal" usage like non-graphics intensive software. The results you get also tend to vary from test to test depending on other factors within your system such as any background programs that are running, how long you've been using your system before running the test, etc. Most hardware review sites try to get around this by only running benchmarks immediately upon startup, and by running multiple loops of the benchmark to get an average score, but it is still a bit iffy in terms of reliability. The final problem is that to overclock your memory, you also have to overclock your CPU, so it's difficult to tell whether the memory or CPU causes the performance increases you get. Really the only differences that can be easily measured are changes in memory latency, since the CPU and memory speed will remain the same during these tests.

There is some debate about just how much of a performance increase will be seen with overclocking, and it depends on how much overclocking one is doing. It is fairly obvious that increasing the CPU clock by 100MHz will give a much bigger performance boost than only increasing it by 20MHz. It is generally safe to say, however, that a moderate overclock will boost system performance (as measured by benchmarking programs) by about 5-10%. Whether this difference is actually visible to the naked eye, however, depends on a variety of factors, like the type of software being used, such as games, graphics programs or video editing software.

The main danger of overclocking is damaging or destroying one's system. Because of the potentially dangerous heat caused by increasing the system clock, overclockers run the risk of burning out the circuitry on their motherboard, CPU and memory, as well as their graphics card if that is overclocked. This can cause irreparable damage to the system, with users needing to replace one or all of these components. Since these components can cost hundreds of dollars, overclocking is not something for the faint of heart. Many motherboard companies have instituted fail-safes that make overclocking easier and less dangerous, such as thermometers that measure how hot the board and CPU is getting, and will shut down the board or decrease the CPU speed if they get too hot. There is always potential for damage, however. In our opinion, the risk of overclocking is rarely worth it unless the person has the money to replace these components if necessary, and the time required to do serious research into overclocking before attempting. For most people, overclocking is a challenging hobby rather than a way to get serious performance benefits, although it is gaining more widespread acceptance as a viable alternative to regularly upgrading one's system to keep up with the latest hardware releases.

Some of the best educational resources for learning more about overclocking are Tom's Hardware Guide at www.tomshardware.com and Anand Tech at www.anandtech.com