(As I understand it) you have not provided enough specific details to answer your question. Let's take it to a bit more extreme. Do you think a 4k mkv will play on an old XP machine? No, of course not, the system was not designed to handled that type of video. So, if the rest of your hardware is sufficient for playing the video in question, then, no, a dedicated GPU will not be of any help. If you are borderline, the GPU may improve the experience, but throwing a cutting edge GPU into that XP machine, will do nothing to allow it to play that video.