There was a ruling some time ago that stated that computer upgrades, no matter what, will not void the warranty on the rest of the parts that you didn't upgrade -- so, for example, you can upgrade the hard drive in your computer, and the rest of the computer will still be covered under warranty.
I don't think that upgrading the RAM on your Mac mini will void the warranty -- however, if you manage to damage the Mac mini in the process, then of course, that damage will not be covered under warranty.