I recently bought a 2013 iMac 27" 3.2GHz QC i5 (I originally bought the 21.5 but after finding out that you can't upgrade the RAM without dismantling the entire computer, I decided the extra 500 was worth it, not just for that, but for the obvious upgrades as well). So it came with 8GB of RAM like all iMacs right now, but I bought it through Amazon (at $120 less cost with no tax as well), and the downside to that is you can't have it customized (as if I would pay Apple $200 for 8GB of RAM anyways; although it would be nice to have upgraded some other components like the HDD but oh well). Anyways, now that I have it, it's up to me to upgrade the RAM, which is no problem as I have done it a thousand times before (not on this model but I'm sure it can't be too difficult). I decided at first to go with 16GB. The 8GB I have already gets rather low sometimes, I tend to run a lot of apps at once, and intensive ones at that. Also I would like to be able to run Parallels/VMWare without worrying about my RAM, so now I am thinking of going a little higher. I had 16GB in my old 2010 iMac, but even that would get down to 4-5 GB of Free RAM unless I made sure to close out a few apps and reboot occasionally. My main question is this: I have heard that installing different sizes of RAM can cause disadvantages. For example: I have 2 x 4GB of RAM at the moment, what if I were to install 2 x 8GB of RAM instead of another 2 x 4GB, would that cause any issues or disadvantages? I know that iMacs take advantage of Matched Memory Pairs, but I don't know if that includes all 4 sticks, or just the two side by side. If it is the latter, I shouldn't have anything to worry about, but if not I should have the same size in each slot, in which case I am going to have to wait to upgrade past 16GB because not only can I not afford 32GB of RAM (even if I sell the 8GB I have) at the moment (especially after spending 1680 on a computer, and I originally intended to spend a lot less with the 21.5"), I don't NEED 32GB nor could I see any situation in the near future where I might need that much. Obviously it can't hurt to have too much RAM, but I am trying to be thrifty yet still make my machine more efficient and powerful, so whatever I am able to upgrade I will, even if it's over time.