are you sure its booted up? and if you are sure, just to check, just leave the video card in there and boot up, trying to isolate if any of the cards are dead. if that is still a problem just put in another card that you are sure that works.
if that does not work, rearrange the power connectors that goes to each device. i had a frends computer, a amd 1700 xp comp that had that problem. all i had to do was to get the power cables in a certain way for it to boot up. but if it wasnt in the right way, then the monitor would stay blank and the computer would juss sit there, doing nothing and not booting up.