500 MHz (DDR2-1000) CL4 @ 2.1v totally stable. 600 MHz (DDR2-1200) CL5 @ 2.06v totally stable.

240MHz is DDR2-480MHz, that is worse than DDR1 speeds, and you would have to be underclocking the FSB to achieve that. Are we on the same page here? What is your cpu FSB set to?