The 17" unit has been sold off and a second identical 21" CRT taken it's
place. I'm trying to use the VGA and DVI outputs of the GF3 card to drive
the two 21" CRTs but all I can manage is 2 identical screens... not much
good to me! :-(

I don't think you can do any better with a GF3. It doesn't have dual
RAMDACs, so it can only produce one output signal. I should know, I
tried it with my old Ti200 a couple years ago.

In desperation I've ordered an FX5700LE 256MB DDR TV OUT/DVI from Tekheads
(after the second 21" CRT the budget is stretched!)... am I going to get the
same problems with screen spanning? (by that point I will have taken all the
old cards out)

That card should be fine. I do dual monitors at work with an FX5200,
and it works fine for me.