sorry I have no idea about the unraid specifics, so hopefully someone else with more direct experience will reply.
otherwise, your primary option is probably an applicable nvidia card, of which depends entirely upon what you are trying to accomplish and how much your budget is.
p2000 are often recommended, gtx 1050/1060/1650 are possibly cheaper alternatives.
if you intend on transcoding 4k content, you should prepare yourself for extra pain.
you might want to read the whole thread @ Plex, 4k, transcoding, and you - aka the rules of 4k - a FAQ
