does your power supply provide ample wattage to your video card? (what wattage is your power supply?). also try to look inside your power supply if any of the cylinders (towers) (capacitors) have bulged tops. if there is no screen and your computer simply shuts down then its 95% likely to be the power supply in some way or another.

Note: high end machines typically require at least a 500 watt power supply to run the fans/video cards/harddrives ect....