We got 2 things here that sound weird:
a) you try to find max clock and atitool lowers the clock?
Maybe your card overheated during findmaxclock, atitool detects artifacts and lowers frequency, artifacts still there -> one step down again, and so on.
In any case: it is very stupid to leave the card alone in such processes as atitool does not have any temperature alerts yet. You are resonsible to keep the temp at it's correct place.
I guess you can be glad it's just a x800 as they are pretty cheap these days
b) I strongly doubt that there can be a damage from underclocking.
Overvolting -> yes sure
But it's not like the unused "volt" burn the card, that's like removing a battery from a device and the battery starts heating up and explodes
Well, we all know this can happen *cought* *sony* *cought* but it's a rare thing
For the future: always keep an eye on the temperatures, make sure your cooler can take the heat in a stress test (the temperature has to stay stable at a specific point during stress test)