Stretch's response is not quite the whole story.
He is absolutely correct that the RAM is the only thing that you can upgrade without voiding your warranty.
If you still own the machine after the warranty period, or have more money than sense and don't mind paying for any repairs it may need in the future, the CPU is socketed (from what I've read) and can be upgraded with a faster mobile core2duo, and the hard drive is a standard 3.5" serial ata desktop drive.
However, you have to realize that these machines were not designed to be upgraded like that and thus, any upgrades beyond the RAM will not necessarily be easy to peform, and in the case of upgrading the CPU to a not-yet-released higher clockspeed or quad core chip, may be so impractical as to be effectively impossible (assuming that it's not just flat out impossible alltogether).