I'm gonna go out on a limb here with the fact that all it is is a small IDE hard drive. I guess if you wanted to you could hook the one currently in your computer up to it, but they have to also have the smaller ones.
Some googling reveals
http://www.digg.com/apple/Replacing_...Step_by _Step
It doesn't seem to hard. Order replacement drive from Toshiba, connect, re-flash firmware.