Just because your card isn't getting hot, doesn't mean it's going to be stable. Going from 800mhz to 900mhz isn't going to get you very far if that GPU isn't going to handle the increase. Try starting at 815 and running Unigine Heaven in a window while watching the meters on Afterburner. Watch for artifacts (irregular textures, missing textures, and usually the most common are small bright and dark specs just popping up randomly across the display on all textures).
If you're not showing artifacts and everything is running great, try another small bump in the speed. You did do a smart thing by only adjusting the GPU's clock and leaving the ram alone. Do one at a time so you can correctly monitor problems.
From what I've heard the 7750 isn't a good clocker in the first place, so don't expect a giant overclock with it.