Here's what I have:
60GB iBook Hard Drive (built in) (18gb Free)
80GB LaCie d2 External Firewire Drive (10gb Free)
15GB iPod (5gb Free)
Here's what I do:
I keep 15gb free on my internal, and store everything that doesn't fit on my internal on my external. This way, I can have what I usually use with me, such as music and such, and other stuff that I don't use as often, or only usually use when I am at home at my desk on my external. If I need to take something with me, I nab it off my external the transfer times are very fast. This set up is extremely efficient, and I have more space than I could have if I had just updated the hard drive in my computer. I would suggest an external drive, you can get small, bus powered, portable, shock-resistant drives from LaCie that are extremely efficient at doing what the do. They slip into a pocket if need be, and can be found up to 80 gigs. Good luck! 🙂