Thanks for the info dosfreak.
Both the ATI and the nVidia that you speak of date back to 2002 -- and time is the heart of the problem. Back then NT4 was still common, and new hardware still worked with it. Now things have changed, chips and drivers are different.
Manufacturers still put out NT4 drivers, but they don't seem to work with the new video boards. (They don't crash, I just get the plain VGA driver after re-boot, because "your new driver is missing or incomplete" or words to that effect.)
I found where it says you need an intel chipset:
http://www.xpertvision.com.tw/en/support/faq.html#4
(My card is from xpertvision.)
"No matter what OS you are using, if you use M/B with VIA, ALi, or SiS instead of Intel chip, you have to install AGP.VXD."
You can't install a vxd on NT4, so you need an intel chipset.
If anyone knows a new GeForce AGP card that really works with NT4, we'd love to hear about it.
But my client has a few old unused Win98 licenses, so these might do for a stop-gap -- leave NT4, without having to buy XP.
Next job after that, put all the genuine Win98 DLLs in WINE, and see if that really works.
(Surely someone knows already, but I haven't found the page where they told us about it.)
noot