The guys at TweakTown have five high-end DDR2 kits on the bench to see which one is worth the premium price you'll be paying for it. They run all five kits on a Core 2 based system using a Gigabyte P35-DQ6 mobo.

"Today we have DDR2 memory modules topping out at 1200MHz DDR2 or 600MHz clock speeds, and this is using only around 2.4v which is within the safe zone. With even more time, cooling and voltage adjustments, more can be possible."