Can't upgrade Ibooks CPU as it's soldered to the mo' board. Powerbook Pismo was the last Powerbook with removable CPU board, and hence has CPU upgrades available.
Best you could do for the Ibook is to max out the RAM to the full 640mb (one 512mb module). Use PC100 2-2-2 RAM, as this is the fastest with lowest latency. OS X love RAM.
Secondley, replace the internal hard drive with a 5400RPM or 7200RPM hard drive....the Hitachi Travelstars 40GNX (40gb 5400RPM) & 7K60 (60Gb 7200RPM) are great performers & quiet, also come with 8mb cache.
Don't know much else that can help hardware wise.
P'z