My iBook with a 74GB harddrive (80gb to the retailers), came to me with 60GB left on the HD. Tiger and all of the software that came included took up close to 15GB of space.
I did a clean instal a few days later and after unchecking all of the printer drivers, language support, Nanosaur etc it amounted to a very small footprint of about 4gb. You can always reinstall some of the software on the OS disc at a later date if you want it too.