Hey welcome to the forum :D
An upgrade of the HD depends on what make you have to start with. If you have a Toshiba HD then you can get a 60GB version. If you have the Hitachi then you’re out of luck because it uses a different connector and Hitachi don’t do bigger 1.8” drives.
I don’t know where you can get the Toshiba drives from, but I know others will.
You don’t say wherebouts in the world you are, just so people give you relevant websites etc