I purchased memory from OWC which is guaranteed to be compatible with my Macbook. I've had some firewire problems with the Macbook and wanted to know if that would void my Applecare or warranty before I contacted Apple.
Since Apple has given out DIY guides to changing the RAM and recommends installing RAM in pairs it doesn't void your warranty (unless you break something yourself while installing the RAM) and it would be strange if it voided AppleCare...
...however: Traditionally your Mac should have at least one stick of original Apple RAM to still be covered by AppleCare...
Best thing might be to just call AppleCare and ask...