Also get it from a good brand: Antec EA/ NEO/ TruePower Series, Corsair, Seasonic, Silverstone, CM Realpower/ Silent PRO, PCP&S, OCZ(Make sure you get the 80+ Certified ones)
^ I've heard of people running HD 5770 CrossFireX on a Corsair 550 W PSU. In reality the whole system with an HD 5770 would use less than 300 W. The extra 200 W is just to be safe.