I would just assume it doesn't, and budget in a new power supply with the card. That's probably one of the easiest things in the computer to replace. Well, unless it doesn't fit in the case, in which case you'll need a new case, too, and then it becomes one of the hardest, only because you then have to move friggin' everying to the other case.
Recommendation: go modular on the power supply.
Me, personally, am going to go with the new Radeons.