the whole thing blocking your from DVD authoring is the G3. you need the G4 to do the mpeg-2 encoding. look at the requirements for DVDSP 1.5
Power Mac G4 computer with Apple-supplied AGP graphics card and DVD drive (configuration must support Apple DVD Player 2.0 or later)
your G3 doesn't have an AGP, or the processing power. your best bet is the processor upgrade. i'm running a sawtooth 450, and it works fine with FCP3, DVDSP, iMovie, and iDVD. okay, it's slow, but it works.
sure, $300 is a lot, but it's a lot less than a new mac, and it should still get you some utility out of that G3. although, i'd still wait to see what comes out next week.
tr