In the Mac Mini booklet, it clearly states that any attempt to open or modify the Mac will result in a warranty void.
If you actually clicked the link I provided, you would read:
"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."