I am running a 5800X, so it's ok in terms of CPU.
I can get a used 6950XT for 595 and a new one for 645, Both are XFX Merc 319, the 6800XT is a Red Dragon.
Checking for 6900XT I found one for 535, but I cannot test any of the used ones, since they are far away. The Spanish secondhand market it's not good at all.
And to go for a 6900-6950XT I would probably need to upgrade my PSU, it's an old 850W (EVGA Supernova G2)