I'm amazed that you see no speed increase in using both machines. Run the client on the G4 with the render manager to set up all the things you want to render. Set up lightning on the G3. On the render manager select the option to optimise tiling, and the option to render on host. This should let your image render different tiles on both machines simultaneously, even if the g3 only does one tile to three or four on the G4 it should still give a speed boost.

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Toolset: Blender, GIMP, Indigo Render, LuxRender, TopMod, Knotplot, Ivy Gen, Plant Studio.