acceleration in apps for video is a relay complex subject.
some parts are simple FCX likes AMD cuards
apple has put the PC RX 560 MSI & RX 580 sapphire pulse on it's "cards to run osx" list and it looks like they will have support.
VRAM rule of thumb
2GB=1080p
4GB=4K
RX 580 8GB is a nice option then,
adobe will run on the AMD cards and it will be faster than your HD 5870
@Spacedust adobe and GPU acceleration tends to be disappointing more than anything else for most users & im relay not shore that "Photoshop, Illustrator, Lightroom" are CUDA based ? IDK where that came from, openGL last i looked.
PP & AE can gain a lot from a GPU but for a lot of workflows you dont need much of a GPU before you hit massive diminishing returns on the acceleration you gain from the hardware
(and the GPU acceleration and the VRAM thing are two different things)
now Davinci resolve is massively dependent on GPU speed adobe for most things loves 4-6 fast CPU cores and a GPU with 2-4+ GB VRAM unless you are doing a more specialized workflow or using some GPU dependent plugins.
but to be fair im a bit out of date on adobe if you want some detailed info on how there products run look here
https://www.pugetsystems.com/all_articles.php
tests on windows but it will mostly hold true for mac apart from nvidia cards are a lot slower on OSX
this one is fun
https://www.pugetsystems.com/labs/articles/Adobe-Premiere-Pro-CS6-GPU-Acceleration-162/
that CUDA scaling in PP
edit extra
FAQ: How do I speed up rendering, exporting, or encoding?
https://forums.adobe.com/thread/2122549
CUDA, OpenCL, Mercury Playback Engine, and Adobe Premiere Pro
https://theblog.adobe.com/cuda-mercury-playback-engine-and-adobe-premiere-pro
Adobe Premiere Pro and Multiple GPUs
https://www.provideocoalition.com/adobe-premiere-pro-and-multiple-gpus/