At the same time, Apple also increased the maximum RAM for the
MacBook Pro from 2 GB to 3 GB. That’s an impressive number, but if
there are two RAM slots, why not a 4 GB limit? When working on my
reviews of the Core 2 Duo iMacs — which, except for the
entry-level model, share the same 3 GB RAM limit — Apple told me
that the Intel chip set used inside had limited memory
addressing capabilities, meaning 3 GB is the most the system can
address. I have to assume that Apple is using the same chip set
in the MacBook Pro.
So the 3 GB configuration is one 2 GB chip plus one 1 GB chip. I dispute Seff’s observation that there aren’t many people who need 4 GB in a notebook — I’d venture to say there are a lot of people who need at least 4 GB in a notebook. E.g. anyone who wants to run Parallels with a generouos allotment of memory for both OSes.