Games aren't the only applications that benefit from the power of a discrete GPU. AMD's and Nvidia's GPUs are made up of thousands of processors that can carry out multiple operations simultaneously. Any application that benefits from such parallel processing — be it an image-editing program like Photoshop, data-encryption software, or a distributed-computing project like Folding@Home or Seti@Home — will run faster with the assistance of a more powerful GPU.
Discrete GPUs can also accelerate crypto-currency mining to produce Bitcoins, Litecoins, and other virtual currencies. Miners have been gobbling up graphics cards based on AMD's latest GPUs, because AMD's Radeon architecture has proved more effective at the task than Intel's Core processors and Nvidia's GeForce technology. Where Intel's fastest Haswell-based CPU — the Core i7-4770K — is capable of processing about 93 thousand hashes per second, AMD's Radeon R9 290X can process about 880 thousand hashes per second.
The argument against discrete graphics
There are drawbacks to discrete graphics cards, with cost being the most obvious. Buying a video card at retail will set you back anywhere from $50 to $1000 or more at the extreme high end (although you can buy a very fast card for less than $400). AMD announced the world's fastest video card just yesterday. The Radeon R9 295X2 features two of the company's fastest GPUs on a single card. It's priced at $1500.
Meanwhile, AMD and Intel essentially give away graphics with their current-gen processors (only AMD's FX series and Intel's Ivy Bridge-E chips don't have integrated GPUs), and the motherboards that support those CPUs have display outputs built right in.
A discrete graphics card also adds complexity to a system. Your motherboard must have an available PCIe x16 slot to host the card, for instance. While this isn't usually a concern for a DIY-er, some off-the-shelf systems might not have such a slot. Or the card might not fit inside the case. Or the existing power supply might not be capable of supporting the card's electrical requirements. All because the PC manufacturer didn't anticipate — or just didn't care — that the end user might want to make such an upgrade.
Installing a discrete graphics card in Intel-based systems can also complicate the use of technologies like Intel's Quick Sync video-encoding engine. Quick Sync is linked to Intel's integrated graphics core, and installing a discrete card might disable it. If Quick Sync is something you can't live without, you might be able to re-enable the integrated GPU, but there's no truly elegant way of pulling that off.
There is no free lunch, of course. Adding a discrete video card will cause your system as a whole to consume more electrical power. That card will also generate heat, which will typically need to be evacuated using a cooling fan that might add a modicum of noise to your environment (there are some passive cooling solutions for lower-end GPUs, but they tend to be more expensive than fan-based cards).
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