I have one of the previous generation MacBook Pro's, a 15" 2.33 Core 2 Duo. I always have heard that the maximum memory that you can use in these is 3GB, that you could put 4GB in and it would work but would only use 3GB and if I remember right report 3GB in the About this Mac section. I have been using 3GB but since a 1GB memory stick reported errors I got a replacement. I read that if you use a 2GB stick it would work as 1GB but work in dual channel mode so would be slightly faster. Since I got a good rebate I ended up buying a 2GB stick and it is 667MHz DDR2 SDRAM. I was shocked to boot up my computer and see it reporting 4GB. Since it's reporting it is it using the 4GB or just reporting it as 4GB and only using 3GB? I know I read all of this when Tiger was out so is this because of Leopard that it's maybe working as 4GB?