Recently picked up a 27″ iMac — the midrange model with 1TB Fusion drive (a hybrid hdd/ssd drive) and 8GB of memory. It became immediately clear that 8GB simply isn’t enough, so I hopped on Amazon and ordered this for about $100:
27″ iMac’s have 4 user-accessible memory slots behind a door on the back of the machine.
By adding the two 8GB DIMMs I bought to the two existing 4GB DIMMs I tripled the memory for a hundred bucks. Not bad!