For DUAL GRAPHICS to work, your onboard graphics must have the SAME SPEED as your GPU itself. That is why the recommended DUAL GRAPHICS card for A8-6600k at most is HD 6670. You are gonna have a lot of micro-lags if your GPU+iGP is out of sync speed.
But to be honest with you, HD 7750 is a powerful enough card that would surpass the DUAL GRAPHICS of HD6670+HD8670D. So i suggest you use the HD 7750 as a stand alone instead. Just disable the iGP of your processor.