I'm not sure how Apple fills them out, but if the past is any indicator, they used 4 2gb memory modules to give you 8GB of ram, so you'd need to replace them if that is the case.
Apple gives you 4 slots total. 0 + 4 GB + 0 + 4 GB configuration.
I put in 2 8 GB rams totalling at 8 GB + 4 GB + 8 GB + 4 GB = 24 GB.
In case you really only want 16 GB maybe you can just stick one 8 GB ram in? As far as I know the rams aren't dual-channel anyways so no harm in trying.
Anybody can confirm if the 0 + 4 GB + 8 GB + 4 GB configuration works?
I'm not sure how Apple fills them out, but if the past is any indicator, they used 4 2gb memory modules to give you 8GB of ram, so you'd need to replace them if that is the case.
They have been outfitting their machines with pairs for a few years now which means 2x4GB for the base 8GB, 2x8GB for the 16GB version and 4x8GB for the 32GB version (due to lack of 16GB modules). This is also shown in the online Apple store (the part where you can configure the additional options/upgrades).
If you buy the base version you can add two 4GB modules since the iMac comes with 4 slots and only 2 of them are populated.