I think it is a matter of deciding where you will be happy and going from there. You are using an e6400 chip, so you have an 8X multiplier. 8X400= 3200. Are you happy at 3200? Will your chip do more than 3200? Do you want 3600? If so, you need RAM that does 450. Practically all decent quality DDR2-800 RAM will OC to 450. You may have to sacrifice timings, but it will do it.
What are the steppings on your chip? That may help decide if your chip will even go over 3.2 and whether you need anything better than DDR2-800.