I went with RAM from Omni Technologies for the same model iBook.
Here's the catch. Omni does not have the link below (to the cheaper RAM) show up in a search on their Web site. You will get the more expensive RAM if you go through the manufacturer/model search. (Test it out.)
Instead, try ramseeker.com to get the URL of the "Generic" Omni RAM.
Omni has RAM that works with the newer iBooks (the ones that came out recently), and our model. However, they also sell cheaper RAM that only works with each model, and not both. This is what I bought.
I believe you want the PC2100 DDR RAM (check your manual).
Here is a link on ramseeker listing vendors that sell it.
As you'll see, Omni has it cheapest, for $105.99.
Here is the link to that page on the Omni Web site.
It's called "Generic" and doesn't show up in their search system. Isn't that nice? I bought a 512MB module months ago (maybe June of 2005), and I've never had any issues whatsoever. I'd highly recommend them for your model. Plus, if it doesn't work, I think they have a lifetime warranty. Check to be sure that you can return RAM if it doesn't fit your model though, but this is what I bought, and I have the same model.