Not a bad idea but the roles should be reversed. The faster processor (G5) should be doing the rendering. The surfing, music and chat can be done on the second, slower machine. Get an older ibook or powerbook like my favorite, the Pismo and use that for your secondary tasks.
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PB G4 15" 1.2ghz - Pismo G3 400 mhz - iBook G3 500 "Jukebox"
"SawSmurf" G4 500 - Quicksilver G4 Dual 1ghz modding...