You just have to power down and remove the side panel.
As the user above has said, it has to deliver at least close to 300 watts.
Older power supplies and some of these cheap ones that are labeled as 500 MAY put out 500 watts, but its all on the 5 and 3.3 volt lines. This is not the voltage most parts use on modern systems. Its all about the 12 volt rail(s) now. The more 12 volt, the better. Modern power supplies get the 5 and 3.3 volt rails from the 12 volt rail meaning you can load up all kinds of 12 or a half and half. In general, this makes a quality 300 watt power supply better then a cheap or old 450/500 watt power supply.
The image above is from my media center pc. It runs a 300 watt power supply with a far more power hungry card(GTX 650 ti) then a 6670, but I KNOW it has the power for it with ease