Power supply requirements fo dual 4830s

[beamed24]

So I have this fancy vision of running dual 4830s in crossfire.

I have a MSI P45 Diamond mobo with either an e8400 or q9450. Not sure which one I will put in yet (buddy works for intel).
I will have 2 DVD burners (The LG's from Newegg)
2 HDD

My question is....
How much power will I need to run the cards? The whole system will be watercooled (Parts already in hand from previoius water cooled build)

Is 850w enough? Say a 4870x2 board, will that be enough?

Thanks for links to help or your own helpful information in advance!

 

[caniba]

There are good deals on PC Power and Cooling 750 and Corsair 750 PSU´s right now, you´ll have plenty with either.