Considering you plan to add a GPU later....

MB: Gigabye UD3 ~9,500
CPU: AMD Sempron 145 ~1,800
RAM: 2GB Kingston/Corsair ~2,200
PSU: Seasonic/Corsair/CoolerMaster
        650w ~3.5-4,000 - will barely take 2 280x's (i wouldnt risk it)
        850w ~9-11,000 - will take 2 280x's + 1 270x
        1000w ~ 12.5-14,000 - will take 3 280x's
GPU: R9 280x ~23-25k (gives you about ~700 kh/s)
        R9 270x ~15-17k (~450kh/s)

I would go with a 280x and add another later.