|Is your psu powerful enough to support it? |
What is your psu ATX 1.0 or 2.0?
What are your amps on the 12v+ rail or rails? You can have a 550watt psu but if the amps on the 12v+ rail are low then it's not going to matter how many watts your psu is putting out, because the psu isn't putting out enough amps for the graphics card to function properly.
Also did you make sure to connect the 6 pin connetor to the graphics card thats if you have one?
Iron Sharpens Iron.