Got a PCI vid card you could try? Sounds like the AGP port is damaged. Thats just a quick guess from what you said anyway.
is the molex power cable on your 9800XT connected to your power supply? make sure it is connected very well.
I think you can rule that out since he has also tried his old Ti4200 and it doesn't work either.
sorry my mistake storm (i was not reading to carefully). it is highly unlikely that both of his video cards are duds. so it it probably the agp slot as you said.