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What constitutes 'the best camera for iMovie 9 and a G5 Mac' is a somewhat subjective issue, which has also been a subject of interest to me! 🙂
This may not be quite up to date but may assist you:
iMovie 9 - cameras supported:
http://support.apple.com/kb/HT3290?viewlocale=en_US
I still use an analogue Hi8 Sony video camera which to continues to work perfectly with the older iMovie 6 via a Canopus analogue to digital converter, a setup which does
not work with iMovie 9.
But if you are using a digital video camera you should not really encounter any major problems with iMovie 9.