A good place to start is
Apple History; they have revisions sorted by year; the white iBook first showed up in 2001 as a G3, and the iBook G4 showed up in late 2003.
Most improvements to the iBook G4 have focused on the CPU and feature set. For example, the very first 12" iBook G4 had an 800 MHz G4 with 256K of L2 cache. The next revision went to a 1 GHz G4 with 512K of L2 cache, which was a huge jump. The next model went to 1.2 GHz, and we're currently sitting at 1.33 GHz. The 14" iBook G4 has always had a higher clock speed than the 12" model, though the speed gap has closed a lot in two years.
Feature set is perhaps more important, because a cheaper iBook may not be worth it if you have to spend extra anyways. The first two iBook G4 revisions (late 2003, early 2004) didn't have 802.11 (Airport) or Bluetooth standard. For the former, you can always get an Airport Extreme card, but you can't add internal Bluetooth afterwards; you have to either get an external USB adapter or go for the newest iBooks (which have Bluetooth 2.0 standard). All iBook models except for the very latest ones also have 256 MB standard, so you'll almost certainly want to shell out for at least another 256 MB of RAM on them. And don't forget the software! You won't get Tiger or iLife 05 if you get an older iBook.
Video has stayed the same on all but the latest iBooks: most have had 32 MB Mobility Radeon 9200s. The current models have Mobility Radeon 9550s, which are important because they support some of the shinier Tiger effects.
If I were going to buy, I'd get the newest one. It's not just the fastest; it has a lot of the extras you might want to buy already packed in, so you may only want to get extra RAM depending on what software you'll be using.
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