First thing you need to do to start overclocking is to change the "Overclock Profile" from "Auto" to "Manual" in the "Jumperfree Configuration" section of the "Advanced" menu in the BIOS.
This is from page 2-28 of the ASUS A8N5x manual. If you take a look at pages 2-21, 2-22, 2-28 and 2-29 it covers pretty much all of the overclocking options you'll need to effectively overclock the chip. Read those pages after reading through the overclocking sticky guide here and you'll be all set.