First a couple of clarifications/elaborations arising from my original post.
One thing I didn't mention is the use of Dragon Naturally Speaking. It's not top of my list of important things, because in practice I rarely use it. But part of the reason I don't
often use it at the moment is that it's unfeasibly slow on this machine. The faster it ran, the more likely it would in future be used more often (for transcribing recorded speech and "training" it by the results, though probably not for real-time dictation and commands). So that's the one existing thing which might justify more processing power. Other than that, more power is just a way of future proofing against later more demanding programs, to put off the day when I have to embark on this process again...
NatSpeak doesn't affect graphics requirements, though. The answer to jojo4u's question is that I did start off looking for a board with on-board graphics, just didn't find one which also met my other preferences/specs. This may have been because I was initially looking at ATX boards, whereas it seems most of the on-board graphics ones are micro ATX. As far as I recall, my initial starting point was just "ATX has more room on it for adding cards later, which is a form of future proofing". I'm not sure whether that's actually going to matter to me though - perhaps not, as long as it's got as many card slots etc as I need for now.
Anyway, back to now. Having followed up lots of clues from all your suggestions, I'm liking the idea of AMD AM2 dual core or possibly 939. So took that as my starting point for a motherboard hunt.
I took as my primary tactic (a) find a motherboard which looks plausible, (b) Google it in conjunction with Linux (or Ubuntu or Dapper/Edgy/Feisty). However, no amount of that has yet revealed a board which is running Linux and not giving someone
problems - even when at the same time the board has been described by others as very good for Linux.
There seems to be a tension as well between "old enough to have been tried out by some Linux people and had its problems ironed out one way or another" and "recent enough to still be on sale". Though there's always the possibility of a second-hand one.
So I'm beginning to think that no matter how much more research I try to do, ultimately it'll be a leap in the dark. The best I can do is to avoid the boards which are giving lots
of people problems. And perhaps I have to accept that the chances are against the system working "out of the box" (as it were), and trust that I'll be able to overcome any obstacles (with help of course).
I wanted to ask pelago, about the ALiveNF6G-DVI, have you got USB running OK?
I did look at the Asus M2A-VM but found this:
(Ubuntu problems for >1 person, a fairly recent thread). So bit wary of that, though it's obviously worked for some people.
And, people in general, a much more basic question: any likely reasons why I might regret choosing a micro ATX board rather than an ATX one, provided it's got the right connections?
Thanks again for all assistance.