As mentioned in the retro hardware thread, I recently purchased a Quadro FX 4000 AGP card for $10.00, and other than a slightly less robust cooler, it appears identical to the EVGA 6800 Ultra that I have. I initially tried it as is, and while it worked fine, it had an issue with water in Half-Life Source and Half-Life 2. I was able to change the card's ID with Riva Tuner, and that solved the issue with the water. I then found that one of the pixel units was disabled, and I was able to make that functional with Riva Tuner (from 12PP to 16, same as the 6800). After making these changes, for some reason Riva Tuner won't overclock the card; if you enable it and increase the clock speed even a tiny bit, it says that it failed the test.

When you look online, by far what you find are twenty year old posts where people were trying to make their 6800 Ultras into Quadro cards, which is easy enough to understand considering the price difference. The core speed is only 25 MHz lower than the 6800 Ultra, so without upgraded cooling any meaningful speed increase is out. I've never tried to change a card's ID with a different BIOS, so I don't know if this would work out risk/benefit wise.