The optimal o'c is the highest clock speed you can achieve which is stable and under 55C full load. You should be able to get 2.6 - 2.8ghz with a good aftermarket cooler.
It's the old 'every cpu will overclock diff.' saying. It's hard to just stick it up to a certain speed and cross your fingers, but 2.6ghz should be achievable without too much work.
AMDZone article
A8N-SLi overclocking
Take at look at the links^^.