If you are going to replace a part, start with the graphics card first. You can get an ATi 3D Rage AGP 16MB (Mac Edition) on eBay for $15-$20 and if the computer will turn on (Fans, Front Light, Drives, red light on the logic board) then the power supply is probably fine. The Power supply does provide the Necessary 25V to drive the ADC displays so if you can run a VGA/DVI output, but not ADC, then it might be the power supply, or the card.