the 27" i7, it has 4gb of ram installed, and 2 empty slots? Is it common to purchase 2x4gb and total it at 12gb of ram? I am considering the imac, and it seems more cost effective to buy 8gigs for $239 from OWC, and have 12 total, than have apple ship with 8 gigs for $200. I am just not sure if you can mix the chip capacities in all 4 slots. 2x4gb, 2x8gb. Thanks ahead of time.