According to Apple's iDVD support page...
733MHz G4 or faster required for iDVD
I was thinking of getting an external firewire DVD burner for my iBook running 900Mhz G3, but I guess I would not be able to use iDVD with it.
And talking about "slowness". I have been doing quite a bit of video editing on the G3, to encode the DV quality QuickTime into SVCD MPEG2 format. They take ages. That kills all the fun of movie production.