Okay, so I see Sapphire brought out a Sapphire HD7750 OC Edition. I recently bought the Ultimate addition which has a passive cooler and by the way, runs really really cool. I've currently got the core clocked at 850mhz(Standard is 800) And the Memory clock at 1152mhz(Standard is 1125).
The card is still running at around 34-35C, when idle. In games it raises to around 44-45C.
Don't forget to change the voltage too!
Can really fry it...
Its just a matter of changing it a bit, test, change, test, change, test.
You want to look for little dots on your screen, those are bad, but can be fixed my changing voltage, or bringing it back down.