Looks like I’ve got the problem (mostly) licked. I installed the proprietary drivers, went to runlevel 2, ran sax2 and now it’s working fine at 1680x1050.
For some reason the display preferences dialog still claims its an unknown monitor, but I can live with that.
Also, when I ran up Konsole I got a weird effect where I could see the prompt and what I typed, but not the output (e.g. an ‘ls’ produced what looked like a lot of blank lines). When I clicked on the title bar all the ‘missing’ text became visible. Might be a problem associated with running Konsole on a Gnome desktop, but I never saw the same problem on my old box. Gnome Terminal ran fine though - I prefer Konsole, but can live with Gnome Terminal.
I might give the new user idea a go, see if that picks up the monitor type correctly if I do a startx (and make sure root still does as well - haven’t tried that since I installed the nvidia driver).