I have the exact same GPU i.e., Zotac 670 AMP! & I'm using the SeaSonic S12II 620Watts Bronze PSU!!
You might wanna reconsider your decision! Have a look here-http://www.zotacusa.com/geforce-gtx-670-amp-zt-60302-10p.html
If you look on the right side where it says "ADDITIONAL INFORMATION", you will see "Recommended PSU Wattage - 600-Watt"
Sure it may run fine with a good quality 550W PSU but it is suggested to get a quality 600W or more PSU from any good brand like Corsair, SeaSonic, etc...