The aluminum MB has a bug - can't run 4 gigs of RAM (no matter what they claim). Wait till this gets fixed before you install; yesterday's patch didn't fix this problem.
http://gizmodo.com/5107267/macbook-firmware-update-doesnt-fix-cranky-ram-problems
Yes, that is if you are buying the upgrade and doing it yourself. However, you can go to the Apple Store and let them do it, that way its full-proof.