I finally managed to fix the computer.
Likely Cause:
Corrupted VGA BIOS
So, what did I exactly do:
I downloaded a X800 XT 256MB BIOS with a newer date and thought that it was just fixed. I copied all the memory timings and Vendor IDs so that the new one would not freeze. The only option I couldn't change was data bus width - should be 128bit, but was fixed at 256 in the replacement BIOS.
This turned out to be able to trash the card, but after letting it cool down a bit, it was able to boot from a diskette. There were various ugly artifacts on the screen, but it accepted my commands. So I reflashed the original vendor's BIOS, except that it wasn't a backup from my card, but a supposedly identical one got off the support website.
It flashed OK.
So, after powering it off entirely and powering up, I soft-restarted it 3 times and the picture didn't freeze! I'm now writing this on it and it seems to work.
So BIOSes can get corrupted by themselves.

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