If your not getting any video then yes try an agp or pci video card. If you can get to the bios there should be an option to select agp or pci-e as primary video.
If the computer will post with an agp or pci video card then i would guess the 5900 video card is bad. Is there any power connections on the video card? Have you reseated the 5900 card?
do you get any beeps at all when trying to power up?
Take the config down to bare minimum, 1 hard drive, 1 stick of memory (2 if ddr2), cpu and vid card.
You can also track down where its getting stuck at by removing the vid card and memory, leaving just the cpu. You should get a memory error beep code if the cpu is working correctly. If you dont get any beeps then reseat the cpu. If it still doesnt work then remove the motherboard from the case and place it on an anti-static bag. Hook up the power connection to the mother board and see if it will work outside the case. If it does then double check the standoffs, making sure you dont have any extra ones.
If it still doesnt work then either the board or cpu is bad. Try another cpu or put your cpu in another computer to test it.