Thanks, Gnome- That's actually what I was refering to, albeit a little clumsily. Is the power source on the 3400 large enough to run a couple of decent cards?
Yes. Basic setup has 375W... that's plenty unless you're running HOT gamer cards. They also offer one with 525W, but not necessary.
I have no particular monitors in mind, to be honest probably whatever the best deal on 2 x 19-22 inchers I can find when I am ready - does it really make a difference?
Yes. You SHOULD consider (1) how many monitors, at what resolution, and whether VGA or DVI, (2) what video cards would you need to run them, and (3) what computer has the mobo to accommodate those video cards. You can't just buy just any old computer, with any old mobo, buy any old video cards and any old monitors and necessarily get the results you wish.