Presario 5000 Picture suddenly died completely (Compaq had no idea what casued it)

The Presario 5000 I left on overnight suddenly died completely. Nothing on
screen, LEDs on monitor show no computer even connected and nothing appears
on boot-up, no BIOS check, no cursor, no Compaq ID: Nothing except a
functioning on/off switch.

I e-mailed Compaq who had no idea what even might have caused it (I
initially thought it might be the monitor itself)

But soon I traced the fault to something to do with the Nvidia board which
came with the computer. I took the board out and plugged the monitor into
the SVGA plug in the motherboard and suddenly the whole thing started
working again.

I would just upgrade or replace the Nvidia board but as a cross check, I
tried putting an old Diamond Multimedia 6326AGP video card into it to see if
it might be something to do with something like the video board controller
and the boot up process now won't get past the BIOS check. It
identifies the card immediately on boot up as a diamond multimedia and then
the computer freezes completely with the cursor on the next line but I can't
do anything with it. I would suspect that there is no proper video card
driver, - but I cant see that the computer is even getting as far as
attaching any driver to any video board.

Is this the indicator of a disastrously bad Nvidia board or is it something
on the motherboard which is stopping it recognising and doing anything with
the identification of ANY video accelerator?

L2Q

Also I presume that the video 'card' on the motherboard has less
functionality or memory than the Nvidia board which came with the computer:
Does
anyone know HOW MUCH? Compaq didn't know this either but said that so long
as I don't use it for gaming (which I don't) it wouldn't make any
difference??????????