Purchased the 27" 4.0GHz Quad-core Intel Core i7 retina iMac and I know it comes with 8GB RAM but I want to upgrade it myself... It has 4 slots correct? So it comes with two 4GB cards installed? Can I only add additional 4GB cards? Or can I add two 8GB cards for a total of 24GB of RAM? I wasn't sure if all 4 slots needed to have the same GB of RAM for better performance? (four 4GB or four 8GB). Ideally I would want to go for the full 32GB, but if I buy four 8GB cards then what would I do with the cards the iMac already came with?