If your thinking you might want to SLI in future, get at least a 650-750 PSU. I run a 750 watt psu with 2-8800 GTX. It's works but I would be a little nervous adding another fan. Some of the ATI cards are well known power hogs. Before long the minimum PSU will be 1000 watts.
Here's whats running on my 750. 2-DVD Rom's, HD, 2-120mm fans, 80mm fan, water cooling pump, 2-8800GTX's, sound card. Probally not much room for more power.