There are millions of threads on this (simple search for powerbook g5), and the consensus is this: if you need something now, buy now, if you can wait a month and a half, do so, because whatever happens at MWSF, it'll be a good thing (either new G5 or faster G4--and if you're not interested in what comes out new, you can opt for the current PB which will be loads cheaper.)
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--B