You may install memory (RAM, VRAM), and other customer-installable parts without voiding your Apple warranty.
"Apple's warranty states:
"This warranty does not apply...if the product has been modified without the written permission of Apple..."
Adding memory (DRAM, VRAM) or other user-installable upgrade or expansion products to an Apple computer is not considered a modification to that Apple product. Therefore, it is not necessary to obtain Apple's written permission to upgrade or expand an Apple computer. While Apple strongly recommends that you retain the services of an Apple Authorized Service Provider to perform any product upgrades or expansions, you will not void your Apple warranty if you choose to upgrade or expand your computer yourself. However, if in the course of adding an upgrade or expansion product to your computer, you damage your Apple computer (either through the installation of, or incompatibility of the upgrade or expansion product), Apple's warranty will not cover the cost of repair, or future related repairs.
Warning: Due to the risk of exposure to potentially lethal voltages, you should not under any circumstances separate the plastics of any computer that includes a cathode-ray tube (CRT) in its design, such as the iMac. Only an Apple Authorized Service Provider should separate the plastics of these computers to install an upgrade or make a repair."
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