DDR2 corsair memory & overclocking

[llama_thumper]

guys, I admit I am a newbie to DDR2 and an overclocking virgin so to say, so I wanted to ask a few stupid questions -

On the following pages there's instructions on overclocking an Intel processor. Specifically, on the page I'm linking to, it refers to overclocking of memory, as I understand it(?):

http://www.tomshardware.com/2006/05/10/dual_41_ghz_cores/page26.html

I'm assembling my system but have trouble getting PC2-800 memory, specifically I would like to have "Corsair DIMM XMS2 1024MB PC2-800 DDR2CL5-5-5-12 (CM2X1024-6400)" memory. However, only PC2-667 memory seems to be in stock where I live (Austria).

What effect is this going to have? Can I, through the BIOS, overclock this memory to 820 Mhz (as they do in the article - see the screenshot?) Does the memory NEED to be overclocked in order to work fine with an overclocked CPU (from 2.6ghz to 4.1ghz as in the article with new cooler-fan, or minimally 3.6ghz as in the article with stock cooler-fan)?

Is there anything else that I'm not asking that I should be asking?

Sorry for the noob questions, but am just starting out

thanks!