No can do dude. The iBook uses what is known as SODIMM RAM sticks, which are basically designed for laptops, and are small in size, and also more expensive. Desktops (including the mac mini) use standard DIMM RAM sticks, which are much larger than laptop RAM.
So, no you can't swap the RAM between the 2 machines. To answer your further questions (as they might come in useful one day):
Yes, you only have one RAM slot. Yes, the iBook G4 has 1 open RAM slot (it actually only has a total of 1 slot, the original 512 is on the motherboard and cannot be removed/replaced)
The max RAM in the mini is 1 GB
Yes, you can upgrade yourself, go to
www.ifixit.com for details on how to open up the mini