Well I'll be damned. We talked a bit, having built Geforce 6800 GT based systems at almost the same time. And low and behold, we end up having the exact same issue.

Geforce 6800 GT output suddenly corrupted

I deduced the VRAM on my card had gone bad. Lacking the soldering skills to replace a capacitor, much less a BGA VRAM module, or a donor card to get said modules from, I used the old heat gun "fix". If the problem lie in the cracked lead free solder used around this time, such an amateurish method has a chance of fixing it. Likely temporarily. If at all.

I realize the "fix" is controversial, and will enrage the Louis Rossman's of the world. If you took your card to a repair shop, and that's all they did, I'd be enraged. But if your options are throw the card away and replace it, or invest $20 in a hot air gun off amazon, I say invest the $20 before you commit to a $100-$150 replacement. Especially since you may just wind up having that problem again with the "new" card in 3 months.

Or maybe stretch $60 for a proper hot air reflow station if you think you're likely to pick up actual board level repair. Better tools never hurt.

I just used a $20 hot air gun. I went on to beat Doom 3, Dawn of War, put a dozen hours into Dungeon Siege 2, and a few levels of Return to Castle Wolfenstein. Truthfully, it's gone better than I though it would. I was thankful just to finish Doom 3, which was the entire motivation behind the build anyhow. I do still hold my breath when I turn the machine on. And I plan on updating my original post for posterity when, and probably not if, the card finally dies for good.

If you have the resources to do a proper repair that would make the Louis Rossman's of the world happy, I'd suggest doing so. If you don't, well, there is always the dodgy method.

Damm Namrok! I did not expect that at all, i was literally searching for 6800GT but didn't get your topic as the result otherwise i'd have replied there instead.
It really puts me off buying another one because it indeed literally looks like your photo's, i also read about the baking method.
Whats more reliable in this? Baking or air gun? And if it fails, what would be your plan for a new GPU given our systems are similar in design philosophy?

I believe the oven method is discouraged because it can taint the oven with toxic chemicals. I went with a hot air gun based solely on that. Both methods seem slapdash enough that I'm not sure one is better in any strict sense.

As for replacements, I keep my eye out for a 6800 GT that's reasonably priced. But if anything prices seem to have jumped about $50 from Dec 2020 when I bought the first one. When this finally goes I may drop down to a Geforce 4000 series card just because I got one free.

Ill definitely try the heat-gun first then, given our cards died in exactly the same circumstances i do have a theory of what happened.
Both our rooms were warm, so the temperature inside the cases would have been hotter than usual. You were playing a game, my CPU is overclocked so it would have generated heat from the file extraction causing heat to build up in the case (I still need to improve airflow with an extra fan). This combined may have heated up the solder just enough to slightly degrade in this manner. So the heat gun and not using the card on these hot days might fix and prevent it in the future. Ideally i'd get the card properly fixed for a long term solution, so i may also try to find if i can hire someone to do it for me professionally if they can do more than just heat the soldering and fix the actual cause.

If you have any aspirations of a proper repair, do not heat gun it. If it goes poorly, it can render the card irreparable.

I've done the heatgun repair on a Playstation 3 fat before and it worked fine. Or rather, I used a hot air soldering gun that is part of my soldering station. Not sure I would try using one of those huge heatguns as you will have a more difficult time keeping it from melting the solder on other components on the board. If the airflow is too high, you can actually blow the components right off the board once the solder melts.

I will say that you definitely want to use some no-clean liquid flux and squirt it in between the board and the RAM chips before you do the heatgun / ho air soldering gun repair. This will help keep the solder balls from shorting together when the solder melts and will also help the solder stick to what it is supposed to stick to.

If you have ever used a soldering iron, it is very very easy to see how much better solder flows and works when using flux vs not using flux.