https://www.amd.com/en/products/graphics/amd-radeon-rx-6800
According to this, a 650 watt psu is required for an rx 6800.
If you mean this power supply, it sounds like you should be good to go
According to pc cultists tier list, that psu is a tier A unit which is one of the best you can get. I’ve got an 850 watt rmx unit that I’m pretty happy with. Someone else might be able to tell you more about the rme vs rmx units. But it sounds to me like it would be good to go.
As far as the rx 6800, performance wise it looks like it’s around a 3070 level maybe a bit faster. I’d say if a 6750xt or 6700xt can keep up with a 4060ti 8gb, I’d say the 6800 should beat it. The 4060ti 16gb may be slightly faster that the 8gb card but I think you’d be on par with it or beating it. The exception would be ray tracing where nvidia would have an advantage for sure. But in terms of bang for the buck value the amd card is a good value.
Found a review on it from 2020 but gives you an idea.
The Radeon RX 6800 is the world's most power-efficient graphics card, better than anything NVIDIA Ampere has to offer. The smaller brother to the RX 6800 XT comes with a smaller heatsink, at more affordable pricing, but retains the lavish 16 GB VRAM memory buffer.
www.techpowerup.com
However if you are looking at used I did come across this card…
Find many great new & used options and get the best deals for ASRock Radeon RX 6800 XT Phantom D OC 16GB GDDR6 Graphics Card at the best online prices at eBay! Free shipping for many products!
www.ebay.com
Within a few dollars but faster card. With used cards though keep in mind you don’t get the warranty but I think that eBay has a 30 day money back guarantee. So if you go that route, I’d run the daylights out of the card when you get it to make sure it’s in good working order before that time.
Looks like amd says a minimum of 750 watts for the 6800xt so it should be fine, but may not be a bad idea to bump up to the 850 watt psu for extra headroom if it’s not much more if you opt for the 6800xt. Just a personal preference so you aren’t bumping right up against the limit.