Memory Speed Below Motherboard Standard Memory Speed

x38 supports DDR2 800 natively
http://download.intel.com/products/chipsets/X38/317310.pdf

I'm not sure what's meant by "natively"
I go by Newegg which reports different "Standard Speeds" for different boards manufactured by different brands. I don't know what Standard Memory Speed means in Newegg's term. So I'd like to know if I can use memory that is slower than this so called Standard Speed and if it will work, and whether there will be a speed compensation or something of that sort

"memory standard" that they refer to will be MAX supported memory.

yes you will be fine with DDR2 800

So really anything below the "memory standard" as newegg calls it is completely/entirely/fully functional and compatible just as well as the max speed?

Thats correct, although to confuse you a little more if you have a CPU with 1066FSB minimum is 533 DDR2 if you have 1333FSB CPU then minimum is 667 DDR2