Well, I found some time to sneak out of the sunshine and do some tests:

(1) adding only "nouveau.config=NvMSI=0 to the grub line made no change. Still bad video.
(2) adding only "video=1280x800@60" to the grub line made no change. Still bad video.
(3) adding only "video=1024x768@60" to the grub line made text after the first video mode change stable!
However, as soon as the GUI came up, the video was horizontally challenged.
(4) the command
sudo apt purge xserver-xorg-video-nouveau
returns
(a bunch of loading modules information text strings)
No Action, xserver-xorg-video-nouveau module not installed
and has no other effect.

(5) My installation has no /etc/x11/xorg.conf.d/ folder in /etc/x11/
There is a file named default-display-manager
Its content is
usr/sbin/lightdm

and that's all.

As it turns out, it's easier for me to change the CPU than to muck further with video configuration files.
Next step is to replace the Athlon TK-57 CPU with a Turion X2 TL-60 CPU (which just arrived in the mail).

I'll post the result.

The command only sets up the desired configuration. X needs to be restarted, or reboot for it to take effect, which cannot happen if the kernel command line is causing KMS to be blocked via nomodeset or nouveau.modeset=0.

Since it is an optional folder, /etc/X11/xorg.conf.d/ may not have been created previously. Creating a file in a folder that doesn't exist implies first creating the folder to place the required file in it.

update: swapped Athlon TK-57 for a new Turion TL-60.

No change. Video still unhappy at 1280x800@60 (nouveau), but ok at 1024x768@60 (vesa).

Clearly the software did something right on one box, but not on a nominally identical box.

I'll live with 1024x768 vesa.