I've got a powerbook G4, 1,25ghz with 1.5 GB of RAM
and i can't say it flies but at no measure it's slow... it's a usable system, but the G4 shows it's age....
in my opinion the whole G-series are kinda behind on intel quite a bit.... but with enough RAM you can have a good system
provided you're not into anything demanding (like aperture or other big memory/cpu stuff)
hope it works out for you