[brazen1]

I am using a GeFore GTX 970 and it can play 4K without any problems. I guess even a GTX 960 or a 1060 could be an option. Regarding the rest of the components, I have a Core i7-5820K (6 cores, 12 with HyperThreading), an Intel SSD 730 series (I made this setup at the beginning of 2015), 16 GB of DDR4 Ram (Corsair Vengeance LPX 4x4GB DDR4 2666MHz CL16 Black), a GA-X99-UD3 motherboard (there are newer ones, like UD4). Since you are using it for other intensive tasks, a 6 core CPU would really be good.

Can you play HDR Clips without any issue ?

I do not have a HDR monitor or TV (yet). I could play back anything without any issues, even HDR samples on a non-HDR monitor.

My understanding is a GTX 960 hardware decodes whereas the 970, 980 etc. had the hardware decoding removed which lowered the power consumption appropriate for gaming.
The 960 was aimed at HTPC users and the 970, 980 etc. were made for gamers. h.264 vs. h.265.

I specifically bought an older 960 to take advantage of the hardware decoding vs. the 'hybrid' decoding in newer cards.
I have read many complaints from users of newer cards not able to decode h.265 perfectly.

Curious since you wrote you can play anything if you actually can?
Please try the test files in the Kodi wiki if you haven't already.
Most interested in 4K (UltraHD) Test #7 HEVC 10-bit 59.940fps (in TS)

http://kodi.wiki/view/Samples