much in the same way you can't use more than 4 gigs of ram on a 32bit system.
wait so if I get one of those boards with 16gb ram support I cant get more than 4 unless I upgrade to 64bit windows7??
(little off topic)
Not specifically Win7, but yes on the 64bit part. More over with a 32bit system you'll see less than 4 as the various system resources that need addressing space will 'overlap' the ram. I've seen some systems running at 32bit OS, with 4 gigs of ram, that windows could only address like 3.2 gigs of it.
Here's a gross oversimplification:
Think of the 32 bit part kind of like a phone number.. I look up the number, dial it, and at the other end I've got some kind of information given to me.. the contents of system ram, the contents of video ram, the state of a soundcard buffer... something like that. With only 32 bits to describe my phone number, I run out of numbers before I run out of bits of information I want to find out. Having 32 bits to describe the location I want to store information gives me a total of 4,294,967,295 distinct locations. This is 4 gigabytes.
Ironically all the 64 bit proccessors now don't actually give you a full 64 bits to describe the ram.. not that you could buy that much anyway. They actually set a limit at 52 bits, but that's 4 petabytes of ram. It'll be a while before we hit that limit.