While not a mac, on my PC I've got things setup like this:
Disk0 - 15K SCSI 73GB, Operating system and applications
Disk1 - 15K SCSI 36GB, Swap file, Temp environment for system
Disk2 - 7200RPM SATA 250GB, Games, Work space for getting jobs done
Disk3 - 7200RPM SATA 250GB, System Images of current and previous operating systems
Network storage on free PIII server which has a small system drive and three 500GB drives. One pair of drives is a mirror set which has all my data. A firewire external enclosure attached to that computer has the third 500GB drive which is for weekly backup and incremental daily changes. After one week, the backup recycles. I want to keep the backup external so its easily portable if need be.
By having my OS on a seperate drive, it makes restores as well as switching between my operating systems a snap because there is not much data on the drive. Also I find the sytem to feel faster. The swap on a seperate drive also makes sense when you're hammering the computer and its swapping. If swap was on the same drive as everything else, it would be much slower when all the ram is used up.
I dont keep any important data on my computer. When a job is done, it goes straight to DVD or on my server. Paid $0 for the server and $95 per 500GB hard drive and $30 for the external enclosure. I'm happy with my server setup so far
Anyway this setup is way too extreme for most but I wanted to share it.