The Hd limit where it exists is 128GB. Any Mac that can recognise a device over 128GB will recognise way more than is currently available. The largest 2.5" drives available are actually 500GB. If a 250GB works, so will the 500. They aren't any good in Intel notebooks apparently because they are physically thicker than smaller drives but you should be fine.

I'm actually going to say that 2GB chips will NOT work in PowerBooks since I imagine that alot of people would have done the same trick in G4 iBooks.