Check to see the heat and power profiles for the fastest iBook G4 drive. Then, compare those to the latest 7200 rpm drives. If you find something equal or smaller, you would be fine in switching.
Tests have shown that there is almost no difference in run time. (A few minutes at most which is a statistically meaningless aberration given the runtimes involved.) Performance, on the other hand, is noticably better.
I'd say your main concern should be the warranty which will will no longer exist after you've totally disassembled and reassembled the entire machine. That's why I tell people to get the 60 gig drive upgrade when they buy an ibook. And bluetooth.