4 - 204-pin PC3L-14900 (1867 MHz) DDR3 SO-DIMM.
Suggest Mac Specialist for memory OWC or Crucial.
https://eshop.macsales.com/item/Other World Computing/1867DDR3S08S/
My apologies spinstoirm.
Alas Down Under the new iMac releases are yet to be posted on the Apple website so the information I posted was wrong. Both models use 8GB 2400MHz DDR4 as standard.
Regarding which brands and models specifically will work better with the 2017 27" iMac, nobody knows yet... Things like latency and other details could affect performance and stability. Past Macs have been known to be quite picky regarding 3rd party RAM
Here's some 32GB configurations using 2 RAM Slots:
$249 - Corsair Vengance that seems to fit the bill CL 16:
$259 - Kingston Hyper X Impact with a CL of 14
$259 - Ballistix Sport LT with a CL of 16
$216 - Crucial With CL 17
Unless I've overlooked something or unless the product page info is incomplete all of these should be compatible with the new Mid 2017 iMac's RAM slots.
Crucial's own website is now updated for 2017 iMac's.
I'm going with Ballistix I believe. I've had Corsair Vengance in a 2012 iMac and it lasted 4 years but did finally fail on me and the RMA turn around time was very long. So I ended up buying Crucial's Mac specific RAM for my iMac and also used Crucial for my MacBook Pro. Haven't had any issues since but then again I've not had Crucial for 4 years which isn't too bad. I've had memory on a PC fail as well and it did so around the 4 year mark so that may be typical.
I've looked here and on Apple Support Communities and see no issues with people using Ballistix in their Mac's and it's made by crucial. On Crucial's website which they've updated to list 2017 iMac compatible RAM it says Ballistix isn't compatible but every listed spec is the same:
Here's the Spec page for Ballistix from Crucial's website (note it's cheaper on Amazon and I've linked it above):
Here's the spec page for Crucial's Mac Memory for 32GB x2:
The only spec that's missing that I can see is if the Ballistix is Single Ranked or not but since it's a gaming type of memory (which means it'll be good for vide editing etc..) I'd assume it's single ranked as well.
Posting this in case I'm missing something and someone could point it out otherwise I'm ordering this one.
The OWC 32GB kit (8Gbx2) is $280 and has a CL of 11 while the Crucial kit is $250 and has a CL of 17.