If you get good quality DDR2-800 you should be able to take it up to 1000MHz if not 1066MHz anyway. You probably won't need more then DDR2-800 even when overclocking a Q6600 as that lets you take it up to 3.6GHz at a 1:1 memory ratio. The E8400 can also get to 3.6GHz with DDR2-800 and can get up to just over 4GHz with DDR2-900, a speed that should be attainable by quality DDR2-800 like Crucial Ballistix or G-Skill HZ.