they are not comparable products, the 6670 is slower and a lot cheaper. something comparable would be a 5770/6770 which is on par with a 550ti and cheaper and uses less power.
It says it can deliver 380watts.It should be fine to run a 6770 or a GTX550ti with the rest of your PC.Although because it isn't efficient at all (80% at 20% load=lol) I would suggest you upgrade it.