It would very much depend on how much you are going to spend. Are you selling the old GPU and getting the "new" GPU second hand? Or are you just straight up buying a new GPU?

And what 6800 exactly are you thinking of?

The difference in performance will be minimal. And something like The Witcher 3 is a very old game, so it's not like you need the performance at all.

I would first dive into the exact issues and figure out if it's actually related to the drivers. Usually the issues with drivers are more with new games, not something old. Keep in mind Witcher 3 has always been a janky game, so a certain number of issues is expected. And this is doubly true when using mods.

6800 xt might match 3070's performance on Linux, but I would not consider it an "upgrade". I would recommend looking up more info on ProtonDB and posting specific questions (if you have not tried that already).

Yes, choosing an AMD gpu will give you other benefits as well, like system stability etc., but make sure your purchase decision is worth it.

If I am not wrong, you can sell you 3070 for about $150-200 and buy a used 6800xt for about $220.

One more thing you can try before buying anything... Try a different OS, like Bazzite or Nobara based on Fedora, or PopOS based on Debian. Different bases can alter performance. I had a laptop with 3060 and it always performed slightly worse on Arch and Debian based systems. For some reasons, the GPU didn't exceed 85 watts, even though the TDP was 100 watts. Only Fedora and it's derivates make my GPU work as intended. But this is a case for laptop GPUs.

Switching might not fix your issues, and might just be swapping for other issues.

Ive been running Nvidia with Linux for over 10 years and its improved significantly over that time. Around the time the 30xx series came out I was looking to upgrade from my 1080ti - I wanted to switch to AMD (because I heard the drivers are better) I got a 6900XT and had heaps of problems and even worse performance in some games than my 1080ti. I ended up returning it and got a 3070ti instead, and I had no problems and performance was great.

About a month ago I wanted to upgrade my 3070ti and give AMD another shot with a 9070XT - its been absolutely flawless this time around.

All this is to say there are no guarantees going AMD will solve your problems, but I would consider going an AMD 9070 series instead - particularly since their FSR Redstone thing only applies to the latest cards

I have had great luck with a 6600XT myself, but your mileage may vary. There seems to be a fair amount of variance in terms of which AMD cards have solid footing in Linux and which games they work well with. I haven't had any issues but I generally don't play visually demanding games.

Also, if you ever want to roll out your own local LLM, you're just going to have better performance with an Nvidia card, as ROCm just seems to not be quite up to snuff at speedy work.

I don't know AMD CPUs that well but could it be you are CPU bound?

I have the same gpu RTX 3070 with an i5 8600k and Im CPU bound, not by much though. Last time I played Witcher 3 it was running very good only in Novigrad it was stuttery due to the amount of characters

Its not worth switching GPU for this. 3070 should be plenty capable of running witcher3. Ive got a friend running a 3060 and she plays all kinds of games with very few driver issues.

We should troubleshoot your issue first and see if a new GPU is required.

I went from being annoyed with my 3080 to being extremely happy with my 7900xt. The driver issues no longer became an issue because AMD. One important detail is this was a new build which meant I also went from a 3700X to a 9800x3D.

Personally it's worth it ESPECIALLY since gpu prices have come down.