also 2 gtx 570's will be much better then one 580. however you could get the 580 and possible get another one later. i have 2 570's myself and they max everything out at 1920x1080 and thats even with mods added to the game which make it more demanding. playing crysis 2 with a few mods the lowest fps i saw was in the 40's. otherwise its 60 or higher
PSU: its an OCZ i think 850W or 750W but its 3 years old should i get a new one ??
Nvidia calls for a "Minimum System Power Requirement" of 600W on the specs page for the 580, and they say it draws 244 W. You should be OK until you SLI (if it's not too long), when you'll want to refer to Nvidia's SLI PSU chart, which will have you in a 1kW.
what i have.. mobo- asus p8p67 pro
cpu - i5 2500k overclocked to 4.2
gpu - gigabyte gtx 570 (2)
ram - 8gb corsair vengeance
optical drive - lg blu ray
hard drive - wd caviar black 7200 rpm
psu - 800w corsair
one 580 should be able to max everything out or atleast come very close. if you plan on getting 2 580's then i would look into a 1050 or higher wattage psu. make sure its from a good name brand such as corsair, cool master, sea sonic