I would have to say that a single-card setup would be a bit better. It takes less power, generates less heat, and generally has less issues that a 2-card setup. In the future though, you could get another card of the card you do end up with.
Now, we have to address your processor. While it is still considered quick, its going to have its limit when it comes to very high end cards. You could maybe go for a 7850 or 7870 and get a new power supply: