Inside the MacBook Pro's 3GB RAM limitation

One of the big new features in the Core 2 Duo MacBook Pro is the new 3GB RAM maximum that it supports. But 3GB is a funny number for a machine with 2 RAM slots. Why not max that puppy out with two 2GB sticks - for a total of 4GB?

MacFixIt has learned that it's not a physical space limitation and that both of the new MBP's RAM slots are large enough to accommodate the bigger 2GB modules.
Although the Intel 945PM chipset can physically handle 4GB of DDR2 RAM, there is the potential for "memory overlap" when more than 3GB of RAM is installed.

a number of items that must be stored in physical RAM space, and when RAM reaches 4 GB, there is some overlap. In other words, in a 3 GB RAM configuration, there is no overlap with the memory ranges required for certain system functions. Between 3 GB and 4 GB, however, system memory attempts to occupy space that is already assigned to these functions.

An interesting addendum to the RAM story is that Apple's 3GB RAM upgrade is actually competitively priced compared to what you find on the open market. Apple charges US$575 to upgrade from 2GB to 3GB of RAM and a 2GB SO-DIMM sells for US$625 at NewEgg.com.