Which iBook is it?
How do you know your internal RAM is 256?
There is only one memory socket, so your options are 128 built in + 512 = 640 or 256 built in + 512 = 768.
If it is an iBook G3, chances are your 256 Mb RAM was 128 built in + 128 , the 128 SODIMM was removed to make room for the 512, so you now have 128 + 512.
What does Apple System Profiler say?
Thanks
Trevor
CanadaRAM.com