Memory Help!!!

Hi everyone, this is my first post. Hopefully there are lots of people who can answer this question.

I'm currently running 2GB of DDR400 memory modules in an MSI intel 865PE chipset P4 board, getting on a bit now, and ive been keeping tabs on the numbers getting leaked about the new Intel Conroe.
I want to upgrade, but first i want to understand how memory works in motherboards.
forget about conroe for a second:stop: !!!. can somone explain how i run anything higher than ddr400 in a mobo which says it only supports up to ddr400? im assuming this is sometihng to do with overclocking.:confused:
The main reason for asking this is because when i get rid of my current mobo/cpu/RAM combo, i dont want to carry on using generic RAM. If its going to be a high end system, i want rid of crappy RAM.

Ive seen some ddr2,1000MHz stuff from OCZ on overclockers.co.uk, but how would i run this in, say, one of the as yet unreleased intel965 boards supporting conroe. From what i understand, they are only going to be supporting up to 800Mhz when released. Im guessing the quoted frequency limitations in this situation bears similarities to my DDR400 question. I mean, if ddr2,1000 is on the market, then it must work in something right?!

Im probably being really stoopid and overcomplicating things so any info or links to help me understand this would be amazing, thanks in advance:bowdown:

Some 975 boards work with frequency up to 1066 afaik

http://www.anandtech.com/mb/showdoc.aspx?i=2671

DDR2 1066 is an OC option on this board

Thanks Hans, interesting to know, i will take a further look at the board, but i still dont understand the whole memory compatibility thing,
1. So, could i just put DDR2,1000 or 1066 modules into that board, and it work straight away?
2. What would i have to change?
3. How did you find out that it accepts 1066 as an OC option?

Many thanks