My 2010 imac is shared between four family members and running Yosemite. It currently has 8GB of memory. It seems slower to boot up and also very slow when switching between accounts lately. I have over 600GB free on a 2TB drive. The mac is mainly used for web browsing and photo editing. Nothing cpu or memory intensive. But it seems like I'm getting the beach ball a lot lately, in Finder, Itunes, etc. This is my first mac so I don't know what the lifespan is. I don't want to waste money upgrading the RAM if it will die in a couple of months or a year. Even if I were to upgrade the RAM, should I go for 16, 24, or 32 GB? If I purchase the RAM, can it be used for the retina imac in case the old one dies? According to Crucial, the 2010 model takes PC3 10600, 1333MHZ while the retina imac takes PC3 12800 1600MHZ. I have no idea what the numbers mean though.