To be honest, there's very little you can upgrade in a notebook. But do yourself a favour, make sure your new hard-disk ain't a Serial-ATA, cause it'll never fit!! Instead buy a PATA/IDE version...
Oh, having said that.. did you know that Western Digital make a very nice 250Gb IDE drive which you can install into your PowerBook?
FYI:
Western Digital Scorpio®
Model:
WD2500BEVE
2.5-inch EIDE Hard Drives
250 GB, 100 MB/s, 8 MB Cache, 5400 RPM
http://www.westerndigital.com/en/products/products.asp?driveid=346&language=en