Any thing that keeps the cpu below 61C, or whatever AMD is recommending as max cpu temp these days. This will vary on how well your cpu overclocks, a good chip will hold the same clock at lower voltage meaning lower heat out put. Your case airflow and ambient temps will also play a big role.
Safe stock cooler overclock? 3.6GHz without a voltage increase if your CPU makes nice with you. Undervolt along with your overclock if you can. Watch your temperatures and don't let them go above 60*C on full prime95 load. I like mine to keep below 55*C, personally.