I am looking to buy a 2011 2.5ghz i5 iMac on the coming weeks. I would like to double the RAM on it to 8gb when I get it. From what I've read, it comes with 4 slots for RAM modules and comes with 2x2GB installed. Now would it be better to buy an extra 2x2GB so overal it'll have 4x2GB installed. Or just install 2x4Gb? Would there be any difference in performance?