What to upgrade for 4K video?

[yonk]

Hi, I have Intel i5 2500k with 8Gb ddr3 with Radeon HD6850 card and 500W PSU.
I'm planning to buy 43" 4K TV (used).

What do I have to change to watch silky smooth 4k movies and 4k youtube videos?

 

[yonk]

Would a newer graphics card be any help?

 

[yonk]

The 4K TV will just replace my 24"monitor, I'll use it for watching movies, videos and do regular computer work, like browsing, mail, maybe some light gaming..., just regular stuff. I don't have a blu-ray player in mind.

So it seems that a basic card like rx470, rx480, rx570, gt1030 would do for my needs?