I wasn't sure... I just got a new 27" and ordered it with 8GB from Apple (2 x 4 GB sticks). I ordered an 8GB stick of Crucial memory from Amazon. I have it installed and OS X shows 3 of 4 slots being used with 4GB, 4GB and 8GB. would it have made my system faster if I bought two 4 GB sticks instead? giving me 4 x 4 x 4 x 4. as a side note.... I ordered a BTO i7, 8 GB RAM, 1 TB Fusion, 4 GB graphics and had it shipped to the Apple store. After I picked up and turned it on for the first time, the fans went on at full blast within 10 seconds and would not turn off. I took it back immediately to the Apple Store. They looked at it over night and told me they were just going to do a return because it was dead. So to do a BTO I was going to have to wait another 10 or so days.... or not! I asked what they actually had in stock for iMacs, and he looked on his inventory app and they had my exact configuration except with a 3 TB Fusion. He told me he felt bad about getting a bum iMac and sent me home with the upgraded model at no extra charge!