If you correlate the "Southern Islands" features between ^ those two the 7550 card is 1080p max so it's not going to improve on the existing card that you have. I would also avoid the nVidia card (unless it's $free) because there is zero support for nvidia cards in the Generic/GBM image (our long-term technical direction) and whilst Generic-Legacy still exists and still supports them under X11 and it will do 4K, most 4K content is now HDR and X11 means no HDR support.
The best upgrade would be to a recent-ish Intel motherboard with integrated graphics, as those are going to support HEVC + 4K and most of the recent ones do HDR too (although HDR support is still work-in-progress in Kodi). I can't advise anything specific though as I didn't run an Intel CPU device for LE/OE use in the last decade (only ARM boards) and Intel breeds "lake" codenamed chips faster than I can keep tabs on the damn things.
The best upgrade would be to a recent-ish Intel motherboard with integrated graphics, as those are going to support HEVC + 4K and most of the recent ones do HDR too (although HDR support is still work-in-progress in Kodi). I can't advise anything specific though as I didn't run an Intel CPU device for LE/OE use in the last decade (only ARM boards) and Intel breeds "lake" codenamed chips faster than I can keep tabs on the damn things.
Minimum Kaby Lake for decoding HEVC in Linux. Won’t encode though. Later GPU required for that. Sky Lake did HEVC decoding in Windows using a hybrid driver but that wasn’t ported to Linux. For LE definitely an ARM processor to maximise graphics output performance.
