the power consumption difference of HD7750 and HD6770 is 40W - so HD7750 generates less heat as well
at stock the performance difference is neck to neck - also keep in mind that HD6770 has 50Mhz and 100 Mhz GPU and memory clock speed advantage
AnandTech - Bench - GPU12
but as HD6770 needs additional power ( through a pci-e power cnnector ) it can offer and can be OCed more compared to HD7750 and HD5770 both - so if you are only looking for performance only then HD6770 is what you need but you may need to OC the gpu to feel the difference with a HD7750.
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