MorphOS is extremely fast but you will hit a wall with available software and the whole feel is very retro (plus you have to buy a licence which problably exceeds the cost of you iBook). Linux flavours are an uphill struggle to configure correctly and once you get there, you're still no faster than OSX (and slower for a lot of things).
Your best option is to consider your requirements and stick with OSX - Tiger if you can live with it or Leopard if you require extra compatibility. Either way, you will be able to optimize the OS to get the best from your iBook. I take it yours is the 1.42Ghz machine? With plenty of RAM it's a very capable PowerPC machine - just comb through this forum and/or ask for help for setting up specifics.