If your are considering buying an iBook Clamshell, I would try to get hold of one of the Graphite or Lime G3/466 SE (Special Edition Clamshells) first. These models have the highest spec of the range (466 MHz processors) and are the only models that have DVD-ROM drives, if you don’t need to play dvds then the indigo model is a good choice.
All the later models can be officially fitted with a maximum of 320MB of RAM, though it has been reported that they can unofficially support 512MB of RAM, Clamshell memory can be purchased from Crucial.