You are talking about a Phenom II x6 processor, i assume. If so then the OC is soley dependent on your ability to keep the processor cool. The processor throttles down @ 50C making any OC that under full load that reaches 50C a waste. In my personal experience with an H50 with a 200x38mm fan running @ 3000RPM i can keep A 3.8ghz OC under full load for extended amounts of time. A 4ghz up to 4.2 ghz OC is very sustainable however if you enter into cpu dependent essences of a game full screen (as most games are played) and the processor reached 50c you can obviously tell that the processor throttles down and the game lags intensively due to the setback.
Hope this was helpful if you need any other help with your x6 pm me