I know the processor only supports 16GB (as per Intel), and there are 4 slots for memory. Back in 2009, 4GB chips were really the most there was, so 16GB total was fine. I upgraded the iMac to 8GB back then (in form of 4x 2GB) I happen to have 16GB lying around in the form of 2x 8GB DIMMs. This upgrade is for a friend, and I don't want to just go take apart my friend's computer only to find this doesn't work. I would only put the 2x 8GB DIMMs in for 16GB. I will recycled the other RAM and leave the remaining two RAM slots unoccupied. Does this work?