Adding RAM memory to my 2011 iMac I am adding more RAM memory to my 2011 21-inch iMac. It currently has two 2 GB memory cards. Can I leave those two cards in place and simply add two new 4 GB memory cards, or do I need to remove the existing cards? Is it OK to mix 2 GB and 4 GB memory cards, provided I stay under Apple's 16 GB limit?
Absolutely. RAM can be mismatched and combined in every way. However, 2x2GB will not be as fast as 1x4GB memory. That being said, it shouldn't really matter that much.