I ahve an OEM pc with no other options except to buy an external FSP Booster X5 450watts independent/supplementary power supply for the "AMD Radeon R9 280X" Only.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817104054&Tpk=fsp%20booster%20x5
http://www.newegg.com/Product/Product.aspx?Item=N82E16817104054&Tpk=fsp%20booster%20x5