We have a late 2012 iMac that we like. But thinking about getting an iMac for the family. Considering a new one (21.5") or maybe one of the ones in the refurb store. Looking at the 2.7Ghz i5. 1. How much faster are the new/current iMacs than the ones in the refurb store from 2013? 2. Can I add RAM easily to the 2013 models? To the new/current models? Any other words of wisdom? Thanks very much in advance.