Recommended wattage for 670 is 500watts (http://www.geforce.co.uk/hardware/desktop-gpus/geforce-gtx-670/specifications) so a 550watt PSU would be fine - as long as:
a - It's a good quality one, i.e. a well known reliable brand such as Corsair and is 80+
b - Youre getting a stock 670, not an overclocked one.