The "
later" better be quicker, or you'll find yourself stranded with your single card with the second identical counterpart nowhere to find. More so if they are MSI custom ones. Ask me, I am a sufferer
And forget price drop, they are usually marginal, since by the current trend they are phased out quicker than that. The excess (huge) power supply will go to waste.
Go for 580 x 2 if you can get the second before they are out of market. Else (for complete peace of mind) 6950 x 2 all the way. Go for the MSI Twin Frozr III version as GWL stated.
560ti x 2 only if you can't live without the Physx.