Video Editing Dilema

Since I have been using the Macbook Pro to edit Video’s I have been using the default iMovie Themes and Editing transitions. I want to kick it up a notch or two and I have the need to create some videos that will be used in a new videocast that I will be doing in January and wonder if it would just be smarter to upgrade to a better video editing suite or just buy plugins for iMovie? So if you are a die hard Apple video editor I would love to get some comments on what you think the best approach is.