Excellent question!

Pretty easy, actually.

The first task was to get the whole suite to install on a G4 based machine.

For that I needed nothing more than some extra hard drive space and TextEdit.

1. 1.   I copied the "iLife '08.pkg" installer from the iLife ’08 DVD onto my hard drive.
   

2. 2.   I right clicked on the new "iLife '08.pkg" and selected "Show Package Contents"
   

3. 3.   I double clicked on the Contents folder.
   

4. 4.   I single clicked on the Distribution.dist file, and hit Command-D to duplicate it.
   

5. 5.   I right clicked on the original Distribution.dist and selected "Open with->Other"
   

6. 6.   I navigated to /Applications/TextEdit.app and clicked on Open
   

7. 7.   I scrolled down until I found the second occurrence of "hasSufficientHardwareForiMovie"
   

8. 8.   I scrolled down to the line after "catch (e) {}" and changed "false" to "true"
   

9. 9.   I saved the file
   

10. 10. I went back up to the iLife '08.pkg file, double clicked on it, and installed the whole iLife '08 suite.
    

After about 20 minutes, iLife '08, including iMovie, was installed.  Wow, that was easy.  Hold on...not so fast.