Are you certain that your iBook can boot OS9? New ones can't, but if your bought yours used then you can still do this. Of course, even new machines can run OS9 through Classic; it's just a matter of actually booting.
If you intend to multi-boot, then it is best to put each OS on a separate partition. OS9 and OSX can coexist on a single partition, but the option-key trick to pick your OS at startup time only works right if each OS is on a different volume. Otherwise, it'll only show one of the operating systems on each volume, and it prefers OSX to OS9.
Virtual PC is not its own OS; it's an application. You can't boot into it, so there's no reason to put it onto its own partition.
Once Tiger is released, there isn't likely to be much of a reason to keep Panther around (unless you're a developer, in which case it's very good for testing). However, you may want to use it for a Linux installation or something.