AX is gold certified and HX is silver. Now depends on what you whant to do in the future. If you want to SLI,Crossfire HX 750 is the best option. If you don't want a second video card than AX 650 is more than enough.
AX is gold certified and HX is silver. Now depends on what you whant to do in the future. If you want to SLI,Crossfire HX 750 is the best option. If you don't want a second video card than AX 650 is more than enough.