The specs state that a 300 watt PSU is required however if you have other hardware that also requires power, they state that 350 watt would be better. Other hardware would be things like a sound card, card reader, more than 2 hard drives etc..http://www.ati.com/products/radeonx800/aiwx800xt/specs.html
It could be that the video card is actually faulty.
You can try to disconnect some of your devices like the cd/dvd drive or the floppy drive then boot up. Less devices connected, more power to the video card.