Computers that normally work with iMovie 4 must also have at least 16 MB of video memory in order to use the Twirl, 3D Spin, and the Far Far Away plug-ins in the Titles pane. If your computer doesn't have enough VRAM, these plug-ins won't appear in the Titles pane.
Additional requirements for iMovie can be found in technical document 93598 "
iMovie 4: System Requirements."