If you have relatively late model PPC PB or iBook models, you would be able to run most of the current available applications efficiently. As my sig shows, I have 4 PPC Macs and I have not yet had a courage to jump into the Intel Mac as there are some serious bugs for those rev. A products. I like my PPC Mac's track record of reliability (PPC Macs are fully matured products before they were discontinued this year) as none of my PPC Macs ever exhibited those problems MacBook Pro and Macbook are suffering from(according to quite a few users).
Every new product launch brings hype and sometimes clouds your judgement and critical evaluations. I am not to rain on the Intel Mac's parade; but, there seems to have way too many bugs for the rev A MBP/MB. My own target for the transition to MBP/MB or Mac Pro would be around the 1st or 2nd quarter of 2007 (1 year of production experience for Apple's contract manufactures in China should help those technical and engineering issues to be sorted out).
The 12" PB is a good machine and is very portable. Because mine has a 1.5 Ghz PPC chip, CS2 and MS Office 2004 runs much faster than on MBP and MB (based on my testing at the Apple stores).