I currently have a mid 2011 21.5" iMac running Mountain Lion with 8G of RAM. I want to exapnd this to 16G but i am not sure of how many slots I have available for extra RAM. Should I get one 8G card or two 4G cards? Can anyone assist?
You have four slots each of which can have 8 GBs for a total of 32 GBs. So you can install 16 GBs by adding two more 4 GB modules. Now you should have two 4 GB modules in the upper two slots. You can install two more 4 GB modules in the lower slots.