Ultimately, there is performance to be gained by overclocking, but that additional performance will typically result in higher temperatures and increased power consumption. If a few tweaks smooth out your favorite game's framerates, however, it's probably worthwhile endeavor.
Keeping temperatures in check and altering a graphics card's clocks shouldn't be much of an issue. Altering voltages can reduce the lifespan of the card, but the exact amount will vary from GPU to GPU. Just know what you're getting into before you start, and let the faster games begin!
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