What PSU is right for me?

Will I need to also upgrade my PSU?
If you have a quality PSU of 750W or greater you should be fine running your machine SLI with two GTX 670's
 
You are describing the power rating not the efficiency ratio. The efficiency ratio is the percentage of energy not lost during the process of power conversion. The higher the ratio, the less power loss during conversion. This also means there is less heat generated during the conversion, since the PSU consumes less power during the process.

Ah, yes, I mean output after conversion, it seem I wrong to pick my word, I should say the "greater certificate, less power loss in conversion".
Thank you for the correction, if you don't give me the correction, people will misleaded by my explanation.
 
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