I think you may be intereseted in Kino
, a nonlinear video editor. It seems to do *most* of what iMovie does, if not quite as gracefully. I tried it on a B+W G3 (300MHz), which was *totally* unusable, but it runs nicely on my G4 466. Combine that with Mplayer and transcode, and you've got a pretty nice system.
(BTW, a YDL binary rpm for Kino is available at freshrpms.net)