My "new" X800 card arrived today, and I put it in in place of the X850.
It found the drivers and seemed to work fine, but it's still crashing exactly the same!
I then did some more experimenting.
I put the X850 back in, and removed the Nvidia card.
I then moved the X850 into the 16x PCIe slot where the Nvidia card normally lives.
1/ It isn't a hardware fault in the card.
2/ The presence of the Nvidia card doesn't seem to affect things.
3/ It doesn't matter which PCIe slot it's plugged onto.
I still suspect a memory issue.
Not main RAM of course, some other low level memory issue that didn't happen with my previous motherboard, but does with this one.
I am using the RAM limitation patch from @rloew, which may have an effect, but Windows 98 wouldn't work at all without it I suspect!