So.... even though I couldn't really afford it I went out and bought another video card. None of the 'spares' I have around are lower spec and I expected the same result. After another fun time dragging the computer out, taking it apart etc. Because of the type of card I bought I no longer needed the extra DVD decoder card, which gave me a tv output (useful for some situations).
So, new card in and installed - everything now working and I don't want to have to open the machine up again. Its takes an age to dismantle the cables, not to mention moving the couch to get to it as well.
Actually - it'll stay until I can afford a new sound card (I need a fairly high spec one and the Audigy I currently have is starting to die).
With a bit of luck this setup should see me though a few more years without much extra stuff required. I don't need a seriously powerful computer - but I do need a decent one.