|OK, you were given a dead computer. You then danced with it for a few hours taking it apart and cleaning all the parts.|
Right so far?
Then you reassembled the thing and it started up and ran for 10 minutes without the agp video card and with one stick of ram and the onboard video worked just fine.
Am I still on the same page as you?
You then installed the agp video card and no video.
So some possibilities are crappy power supply, 4 pin 12 volt connector (2 black and 2 yellow wires) not plugged into the motherboard, bad agp video card, an AGP 2 card (must be AGP 4 or better for this motherboard), you bumped something, or you did something else you didn't tell us about..
Do you hear a single beep at startup?
If you remove the agp card, does it work?