The GDDR5 version of the HD6750 is two tiers higher than the GDDR5 version of the HD6670. The DDR3 version of the HD6670 is two tiers lower than the GDDR5 version. That implies to me that that a DDR3 HD6750 and a GDDR5 HD6670 would be on the same tier. I would think the HD6750 is maybe a tiny bit stronger, but the HD6670 would handle eye candy like AA better. The HD7750 is also around that price, and is a GDDR5 card that would be stronger than either of the ones you list. Is that an option for you?