try running your audio config i dont know how to do it on an g4 but im sure if you look in your audio settings something will stick out about performence.

ive done some searching and found sevral links which might be of use for you HERE's
one that might be of use and HERE's another one that might be usefull

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using just the onboard Audio will cause all sorts of problems and the error message tells you that, onboard audio its not intended for professional use.you should change your buffer settings this will help but once you get the M-audio Firewire it shouldnt be a problem. ..1 gig of ram will help hosting more vst's and smoother operation in your host but it wont really help with the Audio aspect. Ram is used to host more programs it doesnt process the audio. also if your logic track has heaps of audio tracks it could cause a bit of a problem as powerbooks do have a slowish hard drive. only 5400 rpm.can you load tracks into ram ?

i dont use a mac anymore so my info is limited...

hope it helps

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Make sure your buffer size is set to at least 512 samples (in the hardware page of the preferences menu in Logic)...I use 1024 but thats because I have a Powercore firewire.

Make sure process buffer range is set to "medium", I've had bad luck with it set to either small or large.

The built-in audio hardware should be adequate though for playback, I just wouldnt use it for much recording.

Try muting every track and hitting play. CPU meter should be pretty low, yes? unmute tracks one by one, so you can isolate what audio regions or plugins are causing the problem. Check and see if you have a HQ reverb plugin (i.e. space designer) loaded with a very long decay time, as this will eat up CPU power very quickly.