Will a 800Mhz DDR2 work on my macbook that has a 667Mhz DDR2

Hi, I've been trying to increase the amount of memory of my macbook since 1Gb is really poor, but when i went to a store and i've tried to replace my current ram memory (2 memories 550mb DDR2 667Mhz) the guy who work there told me that the DDR2 800Mhz won't work on my mac because it was designed for a 667Mhz DDR2 is that true?? some friends told me that buying a greater Mhz is even better and would also work since it is DDR2 so i don't know who to believe if the guy of the store of my friend :S and can i mix a 800Mhz DDR2 and a 667Mhz DDR2 memory cards in case that i only want to change one slot of memory?

Thanks, i really want to increase my memory, a little help would make me really good for buying the correct ram

 

:O thanks a loooott!!! I'm going to the store right away To make my mac ultra fast haha thanks