Ummm.. I hate to break it to you PYRO, but your iBook has only one socket.
Your only choice is to install one 512 or one 1 Gb together with the soldered-on RAM (128, 256 or 512 depending on the model of iBook)
Choose the 1 Gb.
Thanks
Trevor
CanadaRAM.com
PS. fivetoad -- good advice, completely wrong machine, though. None of the PowerPC based notebooks require or benefit from matching RAM. May want to check that next time.