Is your PB under warranty, because if it is then I suggest you buy a larger HD then have an Apple technician put the drive in for you. That way your warranty will not be effected.
I don't recommend taking apart the 12in PB, I saw one where the person tried to pry the case off to get it open and it resulted in massive structural damage.
Unless you are really good with tools, and you have PB disassembly instructions then I would recommend against a self upgrade.