I currently have four Macs in daily use: a B&W G3, a Revision "D" iMac, a slot-loading imac, and a new (white) iBook.
The slot-loading imac and iBook are partitioned exactly as they came from the factory: One partition, with OS 9 and OS X on the same partition.
The 333MHz "Revision D" iMac, which has the limitation of all early iMacs (it can not boot if the system partition is greater than 8GB), has an aftermarket 40GB hard disk drive in it. That drive is formatted 4GB for OS 9, 4 GB for OS X, and 32GB for data.
My B&W G3 also has an aftermarket hard drive, this one 80GB. It's formatted similarly: 4GB OS 9, 4 GB OS X, and 72GB data.