The first chip which will probably head to Mac All-in-Ones or notebooks is a less-than 40W TDP part with an MXM-based GPU that has 10 Compute Units each with 64 compute cores. The total number of compute units is 640 and the card has 4GB RAM. A sub 40W GPU will give users about 1-1.25 Tera Flops which should be more than enough to power Apple displays. It might even have enough juice to play a casual game.

The memory works at 8Gbps and the card has up to five Display Ports or other video connectors. With 14nm FinFET manufacturing, AMD / RTG can squeeze much more performance in the same power envelope compared to 28nm GCN cards. These R2x0 models are Apple's current flavour.  Radeon R2x0 parts have been around since late 2013 and now is a good time to replace them with 14nm new GCN 4.0 architecture. 

There will be a Polaris 10 based sub 150W TDP part that will be enough to replace the AMD Radeon R9 M395X with 4GB of GDDR5 memory that currently sits in 5K iMac. Such a card can have up to 8GB of memory and can easily drive the highest possible resolutions including the Apple's famous Retina 5K display found on the 27 inch iMac. If you like numbers that is 5120x2880 resolution with shedloads of pixels.  

It remains to see when Apple plans to release these cards. Nvidia did not get any of its products into 2016 Apple refresh product cycle and that product cycle should begin soon.