I plan to upgrade my VDR box, with these two important features :
- reading mkv files => I am using xineliboutput and vdpau.
MKV is just a container for data streams and support is wide-spread. Any half-decent player supports mkv. It's also unrelated to xineliboutput and VDPAU so I assume what you actually mean by mkv is being able to play back HD or greater content using VDPAU for decoding and xineliboutput to get the decoded frames to your tv. VDPAU is EOL. Nvidia currently has NVDEC but I can't speak to how well it's supported since I dumped VDPAU for VAAPI a while back.
- receiving 4K channels => for HD, I am using old PCI/PCIE motherboard + TBS6928 (with CI).
Could you share best hardware cpu/video/card,motherboard/..., & best plugin/driver/... you advice at the moment?
When I switched from VDPAU to VAAPI I retired my old hardware and just spent about $120 on an Intel NUC, and couldn't be happier. I'll never bother with video cards again for htpc/media playing boxes. And also non-usb tuners. Low power/low space is my theme now so my days of building pc's for this purpose are over. You might want to look at other options unless you actually want a pc tower.