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.
how much power supply should be enough for amd fx 4100 cpu with sapphire hd 6670 graphic card
The 6670 does not need an auxiliary 75w 6 pin power lead, it can draw all of it's power from the pci-e slot.
A 300w psu would be sufficient. But, since 400-500w psu's do not cost any more, I would look at them.