I had the same question about finding RAM to upgrade the new 27" myself.
My early research would seem to indicate that there are, again, indeed 4 RAM memory slots on the 27" model, so you would need to buy 4, 8GB memory sticks in order to upgrade to a full 32. Apple sends the base iMac out with 8GB of RAM configured in two slots of 4GB each. Meaning, you cannot use the 4GB memory sticks that come with the iMac when you upgrade to the full 32 if that is your goal. It is my understanding that, for best performance, you need to install memory in balanced pairs, which explains part of the reason that the standard iMac comes with 2, 4GB sticks instead of 1, 8GB stick. I really only know enough about this to be dangerous so feel free to correct me on this if I am misrepresenting anything here.
My other belief is that you can keep the original 2, 4GB sticks in place and add 2, 8 GB sticks in the open slots and still gain the benefit of having 24GB of RAM at half the cost of upgrading to a full 32GB. I did this on one of my older model iMacs--Apple phone support confirmed that this would work. Again, this is a little over my head and I doubt I really needed that much RAM to begin with so never knew if the machine was able to take advantage of the full 24GB or not. It did read that it had 24GB RAM in the "About this Mac" screen. I'd appreciate feedback on the validity of my thinking from those more knowledgeable and wise than I.
The type of memory you will need is 1600MHz DDR3L SO-DIMM PC12800 204 Pin. I see that Other World Computing (a trusted favorite of mine) has this RAM for sale now, the 32GB kit (4 x 8) is $195 and the 16GB kit (2 x 8) is $97.50. Seems like a steal given what Apple is charging for this if purchased from them.