if your going with the e6300 then a ddr2 667mhz will be fine to oc it if you want. If your going with e6600 or higher...i would get ddr2 800 at least.
Just to help you save some money if your going with a tight budget or a all out beast
e6300 or e6400 w/ ddr2 667 = good overclock (you can still get ddr2 800 if you want to give yourself alot of room to oc)
e6660 or higher w/ ddr2 800 = good overclock