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What version of Keynote are you using? The latest Keynote (3.0 or iLife '06) supports wider range of resolution. On my 20" iMac LCD, for instance, it shows resolution choices of 800x600 (4:3), 1024x768 (4:3), 1280x720 (16:9), 1680x1050 (16:10), and 1920x1080 (16:9).

BTW, your PowerBook LCD is most likely 16:10 (17" models) or 1.45 or 1.5:1 (15" G4 models).
 
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