You need to look on the label of the power supply.
You should see a current value (a number followed by A letter) for every voltage the power supply produces.
So for example, you may see 30A in the 12v column - that means that out of 500 watts, your power supply can only supply 12v x 30A = 360 watts on the 12v wires.
Your CPU and your video card are the most power hungry components in a computer, and they're both powered from 12v - so how much power the psu can supply on 3.3v and 5v doesn't matter, how much it can do on 12v matters.
Your processor probably peaks at around 50-60 watts, the motherboard consumes maybe around 10-15 watts (ram included in this amount, each stick consumes maybe 1-2 watts), each mechanical hard drive consumes around 5 watts, each fan consumes around 1-2 watts ... from the 12v output.
So you can simply round everything up and just assume at most your computer is gonna consume 100 watts from 12v output, without a video card installed.
So now you can get any video card, as long as the power supply can supply at least 50-100w more than 100w + video card consumption.... you want some safety margin and you don't want psu running at 100% when you're gaming.
If it helps, here's how much video cards consume (you get 2 numbers, one for idle as in windows, browsing internet, watching youtube videos and the second number is how much they consume when you're gaming)
You can go on techpowerup.com and search for a review for any of the video card models below, and you'll have at the end of the review a summary of performance of the video card compared to other models released around the same time ... so for example if you search for a RX 570 review, you'll see it compared to RX 470, GTX 1060 , GTX 1050 and so on ... but keep in mind performance of cards often improves by a few percent a few months after a review, because of driver improvements.
So see the list below ... as an example, if your power supply claims it can do 400 watts maximum on 12v, I would limit my search to video cards that would not make the total power consumption go above around 320-350 watts.
If you remove 100 watts reserved for processor, motherboard and everything else, this means you have 220- 250 watts for video card....
In this scenario, best bang for buck would be RX 470, RX 570, RX 480, RX 580, nvidia 2060 super, 1660 ti, 1660 , 1060 , 1650 - nvidia cards tend to consume less power, but they're more expensive even used.
My 3060ti founders is struggling on my 3440x1440 ultra wide 144hz monitor. Can’t reach more than 100-120 FPS in most games. Would a theoretical 3070ti be best? Decided against a 3080 because it only has 10gb vram. Had a 6800 briefly but missed out on DLSS and the drivers were unstable so I sold it. Also had a 3070 lol. But the difference between it and the 3060 was so little I couldn’t justify the price difference lol.
AMD has RIS which does the same thing as DLSS (and has for a while,) gsync and freesync are intercompatible on modern hardware, and AMD has encoders even if they're not as good. So Nvidia doesn't really have a massive feature set advantage.
As for how much "sense" the 6800 makes, AMD cannot leave a 3070-shaped blind spot in their lineup. Even though the 6800 XT might have better value, the product matchups need to exist.
Depending on the resolution it won't have that much of an effect. You might be heavily cpu bound if you are at 1080p. I'm guessing you aren't running a OC'd intel chip? Thats the only explanation. You should be seeing 50 percent at least frame rate improvements