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An Overview: What is Over-Clocking?
Over-clocking is the practice of taking a piece of hardware beyond its normal limits. This is done by adjusting the hardware's clock to change how fast it operates. While CPUs are probably the mostly commonly overclocked hardware, other parts of your PC can also be made to operate faster than normal, including your video card.
Most video cards come with a stock clock speed dictated by the manufacturer of the reference design - ATI or Nvidia. Video card manufactures will sometimes toy with the clock speed in order to create premium-grade products. This is referred to as factory overclocking. Further overclocking can be accomplished using downloadable programs. In this case, we'll be using Rivatuner to adjust video card settings. Both ATI and Nvidia now including some form of overclocking software with their drivers, but Rivatuner is still the overclocking utility of choice. The program itself uses few system resources, and it allows you to adjust the core clock, the memory clock, and the fan speed. It also gives you the option to save your settings, so that they automatically over-ride the default settings whenever you boot your PC.
A word of warning - over-clocking isn't particularly dangerous, but any time you change the stock settings on a piece of hardware, there is the chance of breaking something, reducing its lifespan, or at least its stability. Most video card manufactures won't cover an over-clocked card under warranty, so consider yourself warned. Also, keep in mind that video card overclocking tends to gain smaller performance increases than what you would expect from a processor. It is very easy to overclock most Intel or AMD processors by 20%, and even if an overclock fails, the BIOS will typically restore itself to safe settings. Video cards are not as smart, and usually have a hard time tolerating overclocks of over 10%.
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The Easy Overclock
The best place to download Rivatuner is from Guru3D. Once you have the program downloaded, simply install it like you any other utility, then launch the program.
Changing your video card's clock speed is extremely simple in Rivatuner. On the main screen, below the Target Adapter section, you will see a button and the word Customize. Click on the button, and an expanded menu will pop up, with numerous icons. Click on the first one to the left, which should be labeled as Low-Level Settings.
That will bring you to the Low Level System Tweaks menu. From here, over-clocking is simple. Click the box titled Enable Low-Level Hardware Over-Clocking. You can change the core clock and the memory clock. The core clock dictates how quicky the GPU itself will operate. Increasing the core clock will increase the GPUs ability to process data. The memory clock is in charge of telling the video card's RAM how quickly to operate, which helps the card supply data to the GPU at a higher rate. Of the two, the core clock is probably the more important, but increasing both doesn't hurt.
Rivatuner uses slider bars for increasing core clock and memory clock, so simply move them back and forth to adjust your GPU's speed. But be careful. Rivatuner will allow you to adjust the speed of your video card well beyond what will cook your card. Play it safe, and just bump up the sliders in 10mhz increments. Once you've done that, press okay to accept the new settings, then close out of the window.
Now you're back at Rivertuner's main screen. Go to the Customize button and select the option which should be furthest to the right. This will open up Rivatuner's hardware monitor, a series of graphs that will allow you to see clock speed, memory speed, and hardware temperature. Since your computer is only idle at the moment, the graphs won't be showing anything interesting. But running the graphs during tests will give you a log to look at, so you can determine if your card is becoming too warm.
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Testing and Adjusting
Now that you have your hardware overclocked, you should test it to make sure it is stable. For this, you need a 3D benchmark. There are programs such as 3DMark which are built specifically for this, but many popular 3D games, such as Crysis, also include 3D benchmarks which will work equally well. Open your program and let it run through a cycle of testing. You're looking for any sign of corruption. If textures look werid, if geometry seems to be out of whack, or if at any point the entire screen flickers, changes colors, or goes black, then you need to back off of your overclock. Obviously, a benchmark which crashes or freezes is also a good indication that you've reached the highest overclock your video card can tolerate. The most subtle sign of problems are artifacts; dots one pixel in size that don't get redrawn as the GPU heats up, so they stay the same colour and stand out from the surrounding pixels.
Getting the most out of your over-clock essentially involves going through the cycle of adjusting the settings and running your benchmark until you find the highest core and memory clocks you can run without the benchmark crashing or corrupting. As said, I recommend moving up in 10mhz steps, but if you feel impatient, you can increase the over-clock by more with each step. Just be careful, because the larger steps you take, the greater the risk of damaging your hardware. Once you've found the highest overclock you can reach without crashing the benchmark, it is time to stress test. Doing this basically means running the benchmark over and over again, but you can also stress test by running a demanding 3D program, like a new video game. If you run into corruption or crashes, even after a few hours, then your overclock isn't stable.
Once you've found your highest stable overclock, you may still be able to squeeze more out of your card. Go back to the Low Level System Tweaks, then click on the Fan tab. This opens a menu with options that allow you to change the speed of your fan. The higher the fan speed, the cooler your video card will run, and this may allow you to run the card at higher speed than you could before. However, increasing the fan speed will also make it nosier and may shorten its lifespan, so you'll need to find a compromise between speed and noise. You'll also very likely reach a point where increasing the system fan has no effect on how high you can overclock your card.
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The Final Step and Beyond
If you've found an over-clock which is stable, and which operates under a fan speed which you find acceptable, then go back to the Low Level System Tweaks menu and click the Apply over-clocking at Windows Startup box. Then click save. Now your computer will use the over-clock settings automatically whenever it starts.
That is all you need to do to over-clock your graphics card. This procedure can work with both Nvidia and ATI cards, and the steps will be the same. Your results, however, may vary depending not only on the maker of the reference design, but also on the manufacture of the card. Some manufacturers will sell cards with better cooling solutions, which are generally better for over-clocking.
Now, load up your favorite game and enjoy the extra speed!