Which card was installed previously?
What system are you using?
What is the the power supply? Please check the rating label and report back with: Total power and Amperage on the 12 Volt rail or rails.
I'm using an AMD Phenom II X4 955 3.2ghz processor on a M4A78T-E MB. The previous card was my onboard Radeon 3300 HD hence the disable of it and such. My power supply is enermax noisetaker 600 watt.