Sapphire Radeon R9 280X says you need 2x150 Watt 8-pin PCI Express power connectors
does the corsair cx750m have 2x150 Watt 8-pin PCI connectors
PSU - http://www.newegg.com/Product/Product.aspx?Item=N82E16817139051&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-na-_-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID=
GPU - http://www.newegg.com/Product/Product.aspx?Item=N82E16814202137&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-_-na-_-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID=