The only part of the system that impacts your hash rate is the video cards. Those 280x are not a bad choice. The 7950's can do better on a cost/hash ratio but good luck finding any.

Also, don't forget you need a power supply of at least 600W for that system, I would suggest closer to 750W as you will get a bit better power efficiency if your not running at near the max that your power supply can handle.