Oh dilema!, should i buy a gtx 580 or two gtx 560s?

i think either of the two options are ok with a 700 wtts psu,and a i5 2500k, now i know that the sli beats the gtx 580, but im worried about price, heat, power consuption, and i dont know how mutch more expensive is a sli ready p67 motherboard than a standar one, what should i do?, i was thinking about buying the gtx 580, and in the future when they become cheaper buy a second one, but then i would also have to upgrade my psu