Ton answer your question - Yes. Yes you should.

A 970 for example costs around £280 ish. It will max out everything at 1080p with very nice frames. And even at 1440p. So If your looking for stable frames, you can go overboard but your guaranteed to sit at 60+ FPS in most games.

The 970 can also run 4k monitors. But at not very good framrates [depending on the game]

So, yes, a 970 is so worth it at 1080p/1440p IMO and if anyone invests into 4k for gaming, personally, I think they have more money than sense.