AGP driver seems to have done it
If you're going to wait for a new release from nVidia, you might be in for a long wait :-) And besides, if _two_ people suggest that you should just try typing
at the command prompt and see what happens, I think it would make sense to just try it!
Oh well, I seem to have solved my problem by using the FreeBSD AGP driver instead of the nVidia AGP driver, as described in the README of the driver:
Option "NvAgp" "2"
This works for me. Of course, I have no explanation for why the nVidia AGP driver worked in FreeBSD 4.7 but fails in 4.8.