We carry RAM for every Mac. Standard warranties apply to any extras that you add to the system.
All memory has a lifetime warranty.
The base systems are usually sold with just a case, motherboard, floppy drive, and whatever memory or video was built into the motherboard. All other components are selected from a list of BTO options for each computer. You can thus mix and match to your heart's content, putting together a system that best suits your needs and budget. In theory, I am enthusiastic about this concept. Anything that can extend the useful life of perfectly good computers has my support.