Well... Why get a 580 when you could just SLI 470s. The performance isn't bad and it'll run most games just fine on max settings even crysis. The 580 is just too soon to tell anything. Power consumption, performance, temperatures, all these things we don't even know yet. For all you know it could need more power than you have, or it could get even hotter than two 470s. As of now, I would say 470 SLI is your best bet.
If you do, do this route, wait for Cayman. Then Nvidia will probably combat the new card will price drops so if you go with the 470 wait for now.