6-6-6-18 at 400MHz is the best that they can achieve. You could try 5-5-5-15 at 400MHz, but you'll have to increase the voltage to 1.95V or 2.0V. If you leave the voltage at 1.8V, your system will crash. If I were you I'd leave them as is. If you overclock, just make sure that they run at or below 800MHz.