OC my video card

Ive gotton it overclocked higher than it is atm, but my question is, even if it will run is the object find a balance at a lower setting? Not like crashhing,hanging that kinda stuff but I noticed a performance decrease after I went over a certain amount. Temp and voltage were fine. Is there something im missing :-?
Edit: My card is a 512MB BFG 7950
Core clock currently is at: 649MhZ
Memory clock is at: 1.59GhZ
Edit: My card is a 512MB BFG 7950
Core clock currently is at: 649MhZ
Memory clock is at: 1.59GhZ