There are hacks to run the iMovie that comes with iLife '08/iLife '09 on G4s if you want to use it (by default it only runs on G5s), otherwise yes iMovie HD (from iLife '06; made available by Apple as a separate download for '08 owners) is the one you would need. If you upgrade your machine to OS X Leopard then iLife '09 (the last version for PowerPC machines) will run.
as this is a 400MHZ, i would not suggest hacking Leopard on it. iLife 06 is fine on my DP 450, but i would suggest having OS9 and sherlock to speed up searches, as with a 80 gb hd, finder search is SLOW!!!