A 550W PSU is sufficient for a HD7970 (that's what I used with mine for the few months I mined with it). It must be a quality PSU however. What you've got may have a "900W" label on it, but may be good for half that. A PSU built by Seasonic (their own, XFX, some Antec), or Superflower (Rosewill Capstone, 80+ gold EVGA) will be good for what is on their label (and then some). Note that the commonly-available and inexpensive Corsair "CX" and "GS" units are not good PSUs. Initially good for their labels, they contain inferior Samxon capacitors that degrade rapidly.