Each chip is different - there are no precise overclock settings. AMD only guarantees that it will operate at the designated speed and no more. You can check with other people and articles as to what they were able to accomplish, and it might give you an idea statistically of how attainable an overclock you can achieve.
Some references for how to do it:
http://www.xbitlabs.com/articles/cpu/display/phenom-ii-x4-920-overclocking.htmlhttp://www.extremeoverclocking.com/reviews/processors/AMD_Phenom_II_X4_940_920_1.htmlIt looks like 20% is easily reachable and 33% a good possibility.