Reader BW is planning to upgrade the RAM in his 2013 27-inch iMac and asks:
“Can I use an uneven number of RAM slots or should I just buy 2 4GB RAM boards?”
Outside of a few early Mac Pro models, no Mac requires you to upgrade RAM in pairs. Each of the four SO-DIMM sockets on your 27-inch iMac’s motherboard can support either a 4 GB or 8 GB SO-DIMM, for maximum of 32 GB.
Matched pairs of memory can work faster than uneven SO-DIMMs, but you will gain more performance from a larger memory configuration than you will from a smaller, paired configuration.
So if you’d like to keep that other slot open for future upgrades, my recommendation is to go with an 8 GB SO-DIMM to boost your iMac to 16 GB total, leaving the four slot open for a possible upgrade to 24 GB at some point in the future, if necessary.
Further down the road, the best bang for your buck for future upgrades may not be adding more memory. Unless you ordered this model with a Fusion drive, I’d recommend replacing internal hard drive with an SSD. SSDs cost a lot more per gigabyte but a ton of perfor