I have a 21.5" Mid 2011 with 4GB of memory. I'm running Maverick. I use Parallels to run some windows applications. I typically use Firefox with lots of tabs, itunes always in the background. I was contemplating upgrading my memory because I get the beachball spinning quite often and it seems like I need to restart my system every other day to get it running smoothly. I know that my system is currently running 2 2GB DDR3's, with 2 open slots. I know my system supports up to 16 GB, but hat are my options for upgrading?