I do know that most PCI-E cards require extra power (See most ATi Cards) directly from the Power Source.. Were there any extra cabels that came with the card that look like they could plug into the Power Source?
It could be that your Power Supply can't handle the whole comp, and doesn't supply the nessasary power to the 3D Card..
What size (in Watts) is your current Powersource?
What kind of processer?
RAM?
How many HD's?
Are there any Case Fans, and if there are, How Many?
I just upgraded to a AMD64 3500, and a PCI-E.. my old 350W power supply couldn't handle it. Bought a 420W, and runs fine.
If you can, borrow a larger Power Supply from a friend and plug it into your computer.