I don't think getting the single best card you can afford is always the best option. If your budget is $250 or less, sure get the fastest single card you can afford. If your budget is $300 or more, and if you plan appropriately, dual-card setups are usually more powerful. Planning on adding a second card later rarely works out. By the time you're willing/able to upgrade your card is out of production, there are faster cards out already, or both. Then we're back at "buy the fastest single card you can afford".
That being said, I'd recommend 2x7850 over 1x680. Crossfire drivers are a non-issue now, and you'll get more performance for less money.