How many watts does a power supply really need to have to power two 7990s?

[jcarrothers16]

I am working on a build with one 7990 but will eventually add in another for quad crossfire but i am not really sure what power supply i need to get. do i need something like a rosewill hercules or could i make do with a 1000w Cooler Master Silent Pro RSA00? how many watts are really required?

 

[The Q6660 Inside]

This should do very well (powers quad titans).
http://www.newegg.com/Product/Product.aspx?Item=N82E16817494006