Unfortunately it could still be the GFX card, the Ram, the PSU or the MB Im afraid.
When the computer boots up normally do you get a single POST (power on self test) beep?
We need to know this to establish whether there is a case or MB speaker present.
If you do normally hear a single beep try the following test, remove the Ram stick/s and
then power up, if you get error beeps it may suggest the Ram is ok and the GFX card/slot
has the issue, this includes the possibility of the PSU not providing enough stable power,
this is why I suggested trying a known good PSU if you can, having yours tested by a tech
is another option, AGP cards are a thing of the past so a replacement may be expensive and
unnecessary if it is the PSU, if you cannot loan another AGP card try a standard pci type
On a working computer, how would removing the graphics card affect the pc boot up?
The computer would behave exactly as it is, when the system attempts to POST if it encounters
a problem component it simply aborts the boot up, dependent on the system you may get error
beeps or diagnostic leds.
Could it be the mother board thats gone? Would a dead motherboard give these symptoms?
Yes one of its components could, this includes bad Ram or GFX card slots for example.
Testing what you can is the only way to determine what is causing the issue with the free and
easiest options recommended that you try first.