The thing about 8 GB is - theoretically you can install onto a single partition 120 GB drive using another Mac, then move it to the target Mac and it will boot. However, the instant one of the system files falls outside of the first 8 GB for some reason (maybe an OS update, security patch, whatever, and the drive has over 8 GB of other files on it already) the machine will no longer boot. So you must partition to ensure reliable booting.