In my older PowerBooks - I own about 20 working PowerBooks now - I mostly use 4GB normal SanDisk CF cards and I'm very satisfied with them.

For one of my PB 540c I would like to get this:

http://pages.ebay.com/link/?nav=item.view&alt=web&id=261922955509

CF PowerMonster II CF-2.5" SCSI Converter Card

but they are so expensive and the old SCSI harddisk still works.