There is very little that can really be upgraded in a MacBook, and most of these items can be replaced without major effort.
1. Memory - Remove the battry - unscrew metal "L" on the insid of the battry bay - remove old memory hidden behind the long side of the "L" - insert new memory - replace "L" pannel - Insert battry.
2. Hard Disk - Remove the battry - unscrew metal "L" on the insid of the battry bay - pull out drive carrier hidden hidden behind the short side of the "L" - remove old drive from carrier - insert new drive - slide carrier back into place - replace "L" pannel - Insert battry.
3. Optical Drive - Disassembel entire MacBook - Reassemble with now optical drive.
1 and 2 can probably be done yourself - though the new components would not be covered by any warrenty or Apple Care cover.
The MacBook Pro is a bit more involved as far as these upgrades go.
No other components can be upgraded - being hardwired together. The CPU cannot be replaced. Neither can the MacBook Pro's graphics chip/card.
