You may be better off selling one of the 270X cards and moving to an Intel i5 system. A 270X is a decent card on it's own, although you won't be playing on high settings at 1440 or higher on it, and I think the benefits of the i5 and a single 270X is better than the AMD CPU and dual 270X cards.
Or spend the money on the i5, run some games on the system with a single 270X. Then with dual ones. See if the dual cards are worth it. If yes, bit the bullet and spend an extra few $ on a system you'll be happy with. If not worth it to you to run dual cards after you play with them for a bit, sell it and that should pay for the difference in CPU / Mobo cost. A used 270X should be able to fetch $60+ pretty easily.