I have that same motherboard. There is no difference in overclocking between 15 and 18. At least I have not noticed any. You need to change automatic to manual. Then you can change FSB.
You should not use the automatic overclocking settings (10%, 20%, ...). Set your chipset timing to manual. Set timings to what your memory can handle. Go to advanced settings (I think that is the name) and set memory to 320. This will remove memory out of the picture. Then, increase FSB gradually and run prime95 until it fails. Reduce FSB until you can runh prime95 for 20 minutes. This is the max for your CPU. You can increase VCore a notch and repeat.
Then, go back to advanced settings and change RAM to 400. Now set the FSB back to 205 and gradually increase FSB and run prime95 until you fail. You can increase the RAM voltage by a notch and repeat.
Now you can decide if you want to run in synch 1:1 (400) or at a divider rate (320).
Now, you need to run memtest overnight. If it gives any errors, you need to reduce the ememory overclock or further increase RAM voltage.
After your memory is stable, you need to run prime95 overnight. If it fails, you need to decrease CPU overclock or increase VCore.