Get a Hitachi TravelStar drive, 80GB (the biggest made for notebooks), preferrably a 5400 or 7200 RPM model.
That said, installing a hard drive in any iBook or aluminum PowerBook is very labor-intensive -- is it worth it to gain just 20GB?
I'd instead suggest using an external portable FireWire drive (such as a LaCie Pocket Drive or OWC Mercury On-The-Go drive). For file storage, such a drive should be perfect (and can be powered from the FireWire bus, for battery operation).
tooki