If you are postive that you will be using just a single video card, then a high quality power supply in the 500 to 550 watt range will meet or exceed your needs. A good choice would be the Corsair VX 550. It can handle any single video card made and then some. Another good choice would be the Antec Earthwatts EA-500.
If you were a hardcore gamer into serious overclocking and mutiple power hungry video cards, then it would be a different matter.