i added 8 gb to have a total of 12gb. I recommend not buying your RAM via apple, because it's way overprized. The installation is very easy and apple even specifies what kind of ram you should buy.
apple doesnt even officially support 32gig for a imac so buying trough apple isnt an option. the question is do you really need the 32 gig.and especially for that price!!
I considered it for a moment but it ended up being too much money for me. I'll definitely grab it down the lines as a means of life extension but at the current price I just couldn't do it and went for 16.