The PowerBook G4 (1 Ghz Power PC, with 1.25 GB of memory) that I've been using for work has no partitions. When I tried to update the operating system from OS 10.4 to 10.5, the installer indicated there insufficient space available for the 6.5 GB installation. I checked, and found only 5.8 GB free. That was with iDisk on.
After turning iDisk off, I checked again, and found 6.58 GB free. However the OS 10.5 installer still indicated that a standard installation of "Mac OS X cannot fit...." So I decided to customize the installation by deselecting Printer Drivers, which require over a GB of space on installation. That sufficed to get me past the first hurdle. The Installation DVD is now checking itself for consistency, a step that is possible to skip....
The installation finished, prompted a reboot, and prompted me to update software. A complete update would have required an additional 2.1 GB - unavailable. In the detailed list, I deselected everything but the security update.
Wow, finally! No wait. Another reboot launched a SetUp Assistant. The assistant got my MobileMe address right, accepted my current password, but then choked on the Japanese location and telephone number. Though I chose English as the main language to use prior to installing OS 10.5.6, the assistant had auto-generated my location, possibly by retrieving it from Apple Japan, in Japanese characters. That and a Japanese telephone number associated with my MobileMe account choked the assistant when I clicked to continue registration. Long story, short: Clicking "Continue" (anyway) in a intervening dialog box seemed to have aborted the registration, because the assistance routine terminated with a reminder to register.
Stay tuned for selective software removals to free up more space for system software and native accessory updates.