Mine is Crucial I bought from a local company that is also on Amazon called Komputerbay. OWC offers 32GB kits but wants nearly $300 last I knew, I got this kit for around $100 but pulling recent results from them on Amazon the prices seem to have gone up a bit. I have had no issues or glitches and being I run a Crucial SSD, I even have my Mac tweaked to use the RAM for many items normally run off the hard drive to reduce writes, like SWAP, etc.
I wouldn't call it over-kill but I am an I.T. Professional so I believe in having maximum power. I use the latest Apeature all the time for Photo Editing and have no issues. You can always try 16 and then go to 32 if you feel you need it. It will cost you around $60-$70 for 16GB and then another $60-$70 for the second 16GB to get to 32GB.
http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias%3Daps&field-keywords=32G B+DDR3+for+iMac
I am not affiliated with them in any way and I have bought several kits from them for Macs and only had 1 out of 20-30 sticks bought for myself, family and friends that went bad. I searched and found they were local here in GA and not far from my home and drove over, they swapped it out no fuss and the replacement was good to go.