Memory function and overclocking

Hey people,

my motherboard says 1600MHZ is overclocking (M4N68T PRO). but I dont want to overclock. I have heard that if you just enter the memory into the slot without trying to overclock it uses at the highest it can? so 1600 ram being used at 1333 max in slot. is this possible?

I may upgrade my motherboard in the (relatively near) future which will have a higher speed, which is why Im asking

Thanks

PS is there even that big a difference between using 1600 and 1333?

tx