If previously using an integrated video device on the computer:
Make sure that the power supply meets the minimum requirements of the video card.
More important than the wattage will be how many Amps are available on the 12v power rail of the power supply.
You can check this on the power supply unit itself by removing the side of the case. The label on the power supply will list the 3v, 5v, and 12v power rails.
If your PSU capacity seems adequate, you may want to check for voltage fluctuations. Please see our FAQ here for further instructions