I have tried two DVI video cards designed to work in an older AGP slot - the ATI Diablotek and the Matrox G550. Neither will output DVI, but will output VGA. In both cases, I updated the drivers as recommended.
I have updated the system bios, and checked the cables/monitor on another system (Mac Pro). I have my doubts that both cards can be faulty, but I've run out of ideas. WHat is it about an older system like this that would prevent these graphic cards from working in DVI mode? Any suggestions?
In most cases that this happens the monitor has received corrupted signals.
Turns out the device ID information in the monitor became corrupt somehow. The monitor sent that corrupt information to the video card during the display driver loading, video card got confused and switched to VGA output, screen stops receiving signals.