(2) 5870's - YES
(2) 5870x2(IE 5970)'s - Not sure yet, we will have to wait on the reviews and benches. But My guess is "maybe", but a 1000W would probably be the best route for this.
My thought is if you have the money for (2) 5970's which are PROBABLY going to come out at a price of $800+ you can pony up for the best PSU you can get. And there is NO SENSE in (2) of those cards unless you are planning to run 3 displays for gaming.