The clamshell Ibook was delivered with MAC OS8.6 when it was introduced in 1999, and the expandable main memory was 32 MB at the time. Later clamshells from 2000 onwards came with OS9.
In principle, OSX 10.3 (Panther) can be installed on all iBooks with a 128 MB memory expansion, but a 256 MB memory expansion is recommended.
The last Clamshell Ibook series with Firewire port even runs with OSX 10.4 (Tiger)
With the help of the Xpostfacto shareware program, OSX 10.4 can be installed on an iBook without a FireWire port.
As a workaround, you can also install a hard drive that already has an installed OSX Tiger on it.
If you don't have much money to invest in your clamshell Ibook with a 3 GB or 6 GB hard drive, it's best to stay with OS9.