HD 6670 is not meant for gaming but still it can play most of the games on low reso. and medium settings.
HD 7750 is best among three of them.
HD 7750 is slightly faster than HD 6770 and performs well.
Even HD 7750 requires much less power than HD 6770.
But if you can save some money and pay for GPU then you should go for HD 7770.
although there is not much difference between the price of them two but HD 7770 is better than HD 7750 for just few bucks more.