I would say definitely yes, especially considering the special upgrade pricing going on right now. I think the audio editing/enhancement capabilities of Soundtrack Pro plus getting DVD studio pro are easily worth the $199 it costs right now.
Plus there is the need to stay up-to-date with your software in general. Even if there is nothing that you especially need in version 5, when version 6 comes out, there very well may be something that will be essential for you. You never know what the upgrade options will be at that time. Apple may not offer and upgrade from the version you have now and you'd have to pay the full price. Moving up from FCS 5 is sure to be the cheapest way to upgrade in the future.
EDIT: Also, the ability to keep the program data files of Soundtrack, Motion, and DVDSP are very useful for those of us with laptops, and i see you're working with an iBook.