Minimum 400 Watt for SLI mode system.
(Minimum recommended power supply with +12 Volt current rating of 24 Amps.)
When it comes to power it is never recommended to cut it close. I recommend getting no less than 550w but 600-650w would probably be best. Other things to consider when getting a PSU is the ammount of amps on the +12v rail. As you can see your set up will require 24a. Any quality SLI certified PSU will have this ammount of amps. Another thing that is important is the effientency rating. I recommend no less that 75% under full load.
Some quality brands to consider:
PC Power and Cooling (very high quality but really too expensive to be practicle)
Antec usually tends to be the cheapest of the quality brands I have listed.