Thanks a lot for your help.
This morning, however, the situation has resolved somewhat. The machine will boot as long as the monitor is not plugged in! At least it sounds like it's booting up properly, I get the chime and all the other noises I'd expect, but of course I can't actually see what's happening. And the clicking from the power supply seems to have vanished.
With the monitor plugged in, nothing. Not the least sound or light when I press the power button.
I don't know if taking out the battery overnight and resetting the PMU (yes, not CUDA, my error, sorry) had anything to do with it, but at least at the moment it seems as though the problem is either in the monitor or the graphics card...
So my next plan is to a) plug the monitor into another ADC-compatible machine and B) borrow a VGA monitor to plug into the other connector on my machine...
Then it looks like either a new monitor or a new graphics card, or maybe both since ADC isn't widely manufactured these days.
Thanks again, I really appreciate that people are prepared to help out in an emergency!
[
] ]'>
Cheers,
Ben