I know Nvidia is recommending a 600 watt power supply for a pc system with one of their their brand new GTX480 cards. I did not see a recommendation at the Nvidia site for the GTX470 video card. I have not seen any power recommendations for two video cards in SLI mode yet. SLI power requirements should be published shortly. Tom's Hardware will be publishing an article about using two of the cards in SLI mode. The articles almost always include power consumption and temperature charts. I'd wait a few days before making any purchasing decisions.
The cards run very very hot. The 470 can reach temperatures up around 95C and officially max out at 105C. That is above the boiling point of water. You will definitely need a pc case with excellent ventilation, airflow, and cooling to handle the cards. GEE! I wonder if the card can make a pot of coffee!
EDIT/UPDATE - I just visited the video card section of this forum. There is a sticky (thread) at the top that has a consolidated list of about a dozen technical reviews of the 480 and 470. Some of the reviews have power consumption and temperature charts for two 480's in SLI mode. I did not find any similar charts for the 470 in SLI mode.