The A7N8X has a jumper on the motherboard that switches between 266/333 and 400. Since the 2600 is a Barton with a FSB of 333, set the jumper appropriately. The default is 266/333, so unless you moved it, it is already correct.
Go to the "Advanced Chipset Features" in the BIOS and change the "External CPU Frequency" to 166.