I use a treo 600 with Tiger, no problems at all using Palm Desktop, with hundreds of to-do items, events and contacts, and over a hundred memos.
Don't bother at all with iCal; iCal looks pretty but it's just a toy. Besides being tedious to use, once you pass about a hundred to-do's and events it gets very slow and starts messing up.
DocumentsToGo works very nicely too, you might want to consider that as a way to carry documents with you.