|It would help to know the motherboard model number or the system model if it's something like a Dell or HP.|
The power supply was a good guess but it could also be the motherboard or even both the power supply and motheboard.
I know it's been awhile since it was running but do you or your son remember any details about what may have happened when it quit working?
When you start it up do you get any beeps? Did the new video card replace an older card or were you previously using an on-board video connection? You might want to go back to whatever video connection you had before.
The cd drives should open if they are properly powered even if the motherboard isn't booting.