Sad reality is that a good quality Dothan/Centrino or Sonoma chipset based XP (Intel Mobile) will absolutely cream a Powerbook when running LIVE.
Simple one of life's bitter pills to swallow.
A 1.86ghz Dothan will easily run 20-30 tracks of audio + some 20 odd + VST effects for example.
Mine can easily sequence up roughly 12-16 tracks of VSTi's (depending on the VSTi's used) + a good 20 VST effects plus some 12 odd stereo audio tracks and very rarely go over 60% totall CPU use.
LIVE is NOT Altivec or Dual Core optimised so a top end Dothan / Sonoma on XP runs Live with about the same grunt as a G5 Dual 2.5 ghz Desktop.
Infact the LIVE test carried out on the Abelton forum found the 2.0 ghz Dothan/Sonoma and the AMD based 3500+ laptop processor where pretty much the two fastest and high powered options for Live currently on the market.
The Imac 2ghz 20" model runs alot closer spec wise to the slightly lower 1.6-1.7ghz Centrino's and will give you far more power than any on market Powerbook can.
Until Mac unleash their first Intel based notebooks the only app's that will give you maximum power on their laptops will be Logic 7 and Digital Performer which both have dual core and full altivec optimisations.
If you can wait that long get Live now and worked within it's limitations on your Powerbook.
If not a good quality PM Centrino/Dothan/Sonoma based Intel laptop will give you the best power based perfiomance for Live in a portable , lightweight , long battery life package.
I might add that live is very stable on XPPro infact I've never had the app crash in some 1.5 years myself.