And this I think is the common misconception among the users. Tell me why Final Cut Express and Photoshop would run faster on Powerbook. They are no different than MS Office and a web browser. Just because you are not familiar with certain application it doesn't make them run faster on Powerbook.
IMHO misguided advices are worse than no advice.
This is really an oversimplification, but it's not without a basis in fact.
We're assuming, of course, that a PowerBook is faster because it has a faster CPU clock rate. Since we're comparing Apples to Apples, this is almost certainly the case: an task that requires x amount of time on a 1.2GHz G4 iBook will probably require less-than-x amount of time on a 1.5GHz G4 PowerBook, regardless of whether we're reading a web page on cnn.com or exporting a movie in FinalCut.
Digital video and photo editing applications typically involve large amounts of data and complex calculations (often floating-point math). Word processing consists mostly of memory operations (loading and storing [small amounts of] data.) Rendering web pages is only slightly more complicated; here, the bottleneck is usually your internet connection.
It's not uncommon to have Photoshop filters take several minutes to complete, and digital video filtering, rendering, and encoding can take several hours
to process, even on a "fast" computer. [Most of this time is procesor-bound, ie; the speed of the CPU is the bottleneck, and not the memory, disk, etc.] Displaying a web page usually takes a few seconds at most, most of which is download time. A 10% increase in processor speed makes no noticeable
difference in how fast Google appears; there's no difference between 0.9 sec and 0.8 sec as far as human beings are concerned. If you're a media professional working with PS or FC, though, it can mean several hours of productivity over the course of just a week. For these people, the faster processor in a PowerBook makes it worth the investment.
[Edited for somewhat better clarity]