Dimensioning a new PSU correctly [duplicate]
Solution 1:
The core i5 is a pretty efficient processor, so the Radeons will be drawing most of the power. Pretty much any 500+ watt supply will do just fine. Just make sure it has two of the pcie power connectors for the video cards.
I have a core i5 2500K in an Asus P8P67 Pro with a Radeon HD 3850 and and it usually draws 130-240 watts ( idle-load ). I have a Corsair 550 watt supply and am not sure, but I think it did have the second pcie power connector for another video card.
Solution 2:
You should have a play with this:
http://extreme.outervision.com/psucalculatorlite.jsp
That should give you an idea of how much power you'll need - then you can buy the next most powerful power supply - best to be on the safe side...