Hi all, I am looking at purchasing a 21.5" iMac around December this year/early next. While the current models are tempting, I intend to hang around for an i7 21.5" model. Although ultimately, what are the major differences between i5 and i7, other than it being a faster processor (I assume?) I intend to upgrade RAM to 8GB as well as purchase a 1-1.5TB External HDD for back ups. Thanks!