I'm pretty sure the X800 chip has a temperature sensor on it... I may be wrong, but I'm pretty sure it has.
If you right click on the desktop and go to Properties, and go to the Video card settings, isn't there a Temperature Sensor option to check the temps on the card?
If there isn't, oh well, but I do know that alot of Radeon cards do now.
I'm doubting its temps that caused it though. I mean that Zalman fan, unless it stopped working should have cooled the card down!
It seems more likely under the circumstances, especailly since theres ALOT of artifacting, that part of the chips just decided to blow on you, though its pretty weird, considering the cards still found under the BIOS.
As a precaution though, pull the card out of the slot, blow on it, and then place it back, making sure its tightly fit.
I hope you can get a replacement if its new enough