Video cards makers tend to overly exaggerate minimum power requirements. In the case of the 6670 they say 400 Watt minimum, but I'm sure it would do fine with a 300 Watt power supply. The 6670 doesn't draw a lot of power and it's one of the most powerful card you can get that doesn't require additional PCI-E power connectors. But that being said, the power supply is a vital component that can damage other components if you use a bad one. Get a good brand like Corsair or Antec and if you have the extra money get a bigger one (more Watt) in case you want to change your video card again in a couple of years, this way with more powers it will give you more options in the future.