Nick and all:
Followup on the earlier thread: tkmedit/tksurfer fail at startup with X BadValue
I was not able to get to the root cause of this problem. However, conducting the installation for the latest nVidia display driver resulted in a system that now does run tkmedit (and glxgears, FWIW).
(I was already running a version of the nVidia driver as up-to-date as the ones you recommended.)
So, not sure whether the installation actually installed anything that fixed a previous bug relative to the particular video card, or whether it replaced some software component that was damaged or missing, or whether it replaced some setting in xorg.conf that was troublesome.
Anyhow, progress has now resumed.
Thanks for the suggestion to try glxgears -- I had thought that other software that was clearly running OK was sufficient evidence that video was OK, but apparently not.
Graham