What RAM is installed in your iBook? Is it 2 x 256 MB sticks of RAM?
In the iBooks and 12" Powerbooks, one piece of RAM is soldered onto the logic board and isn't removable. The other RAM slot is free for you to much around with.
If the RAM that's soldered onto the logic board runs at PC2700, then get another piece of RAM that runs at PC2700 or faster. If you get a 1GB PC2100 stick of RAM, the preinstalled 256MB stick of RAM will automatically scale down to run at PC2100 instead of PC2700.
So a 1GB stick of PC2100 RAM will work in your iBook (and only if it's 200-pin SODIMM RAM), but your inbuilt stick of RAM will also be forced to scale down and at PC2100 speeds. If you get a PC2700 stick of RAM, both sticks of RAM will run at the same speed, so no scaling will be required. If you get a 1GB PC3200 (sorry, I just made this number up) stick of RAM, it'll scale down to run at PC2700 speeds because your 256MB of Apple RAM runs at that speed.
The faster stick of RAM will be forced to scale down and run at the speed of the slower stick.