Q: I can't upgrade

Which model iBook? The earliest models "peaked" at OS 10.3.9. This one, for example:

The iBook with firewire was the first that could be upgraded to 10.4. Does yours have a firewire port?

The other possibility is that the discs you are using are specific to a different model. That won't work. You need the full retail version of OS 10.4.

in addition to Eustace Mendice:

You need 256 MB Minimum RAM to run OSX Tiger.

If you have a d DVD Drive the correct installation medium looks like this:

If you only have a CD drive you need the 6 CD Version

Also, prior to upgrading, check out how much available space is on your hard drive.

http://support.apple.com/kb/HT1514?viewlocale=en_US&locale=en_US

I always figure on at least twice the amount of free space recommended by Apple for smoother operation. It is usually recommended by others to keep at least 15% of your hard drive free.

Good luck.