Linux

well...

This sums up my feelings on Nvidia.

AMD, easily. Its literally plug and play. You can even pick some second hand options for cheap that are still solid for gaming such as the vega 56/64 and the RX 5700XT (which is I use). Intel isn't bad so long as you're not playing the newest stuff, my Arc a750 is solid in games like Fallout 4 and Elden Ring. Starfield is complete mess on it. Another thing with Intel is you'll need a distro with a 6+ kernel to get the most out of it.

I have an NVIDIA 1060 and it is such a pain in the ass to deal with. The number of random problems I've had with it has put me off from ever buying a computer with NVIDIA hardware ever again. Save yourself the constant walking on eggshells and get AMD.

I had a gtx1060 when I started using linux, then upgraded to 2060 then again to 2080, they all worked fine without any major problem (except that file system checking at boot sometimes and wayland). Last year I upgraded to RX6800 and man everything just works, no more filesystem checks at boot, Wayland is mu way to go now.

If I have a nvidia card now I would just use, but if I'm buying a new/used gpu it will definitely be AMD.

Definitely AMD. The drivers are actually open source, much better with less bugs and there are no problems known to me. I currently have had a GTX 1070 for the last 5 years, until I've enough money for an AMD card. My setups, especially Wayland based, are riddled with bugs not present on my (Intel based) laptop - which means the only explanation is the NVidia card. The (admittedly: testing on Arch) drivers have broken two times in a year, not making the system unusable but definitely preventing gaming.
On top of that, the 4090 may be 25% better than the 7900 XTX - but it's also 50+% more expensive than the 7900 XTX, which is a pattern which can be seen for every generation and version of GPUs by Nvidia/AMD. Nvidia's equivalents to AMD's cards are generally 25-50% more expensive, with worse performance but better Raytracing and of course DLSS support - oh wait, DLSS 1 and 2 are only for RTX 20 and up, while DLSS 4 is only for 40 Series GPUs. Which means no matter how good it looks, FSR will be the only alternative for almost all players, even those using NVidia cards like me.

Something different: Intel's Arc GPUs would maybe be worth a shot. According to a PC World article, the A770 beats the 3060 even in it's own habitat - Raytracing. It's cheap and gets better with every driver update. It also seems like the Arc GPUs are compatible with Linux fine, though I'd suggest you look up the compatibility with the games you want to play.

I have used an GTX 3070 with no issues for the past 3.5 years, before that I used an AMD Vega 64 with no issues. I think if I were buying a new card and could stomach the prices I'd lean towards a Radeon 7900 XTX.

AMD all the way. Few weeks ago I finally made a switch from Nvida for the first time. I have no problems in gaming. All games that I play run same as on windows.

If I were buying a card right now I'd get either the 6700XT or the 6800XT because they're both at crazy good value for the money right now, especially if you can get one used or refurbished from a reputable seller with a return policy.

No idea about AMD but I have nvidia 3070 and works just fine. I use gnome for gaming and kde works well too. No settings changed, simple archinstall script with nvidia proprietary drivers and steam.

I'm not sure why but xfce multi screen gaming is problematic, haven't dived into it.

Not saying don't get AMD but nvidia has horrible reputation which is kinda unfair nowadays. Their driver has improved a lot.

I'm using Intel Xe embedded graphics and they are suprisingly good. This is probably the first embedded intel chip that can run games. It's basically on par with low-end Nvidia and AMD cards.