A single HD 5970 can consume almost 360w of power when stressed to the limit as the following OCCT test shows:
However, when playing a Crysis Warhead, power consumption is at a more manageable 241w for a single HD 5970. Of course if it is overclocked, then the HD 5970 would consume more power.Sourcehttp://www.xbitlabs.com/articles/video/display/gpu-power-consumption-2010_3.html#sect0
For simplicity sake, let's just say that two HD 5970 will consume 482w total. But actual consumption should be less since SLI and XFire technology is not 100% performance efficient. Meaning adding another card will not automatically double your performance.
Based on the above info, I will agree with ct615
that you should get a 750w+ PSU. I prefer Seasonic since they are basically the premier PSU manufacturer. But you can also choose from PC Power & Cooling, Corsair, Antec or XFX all of whom have their PSU built for them by Seasonic.