My typical partition goes like this:

• Volume 1 : OSX boot volume. 50GB
• Volume 2 : Audio Volume. This does not need to be very big if you don't work on multiple projects at the same time. You should not leave finished project files on active volume. You backup and archive it until you need it again. When you copy it back next time, it will be defragmented automatically.
• Volume 3 : Document Volume. This is where your documents lives instead of Document folder under OSX boot volume. These files are such as Office documents, DAW setup files, iPhoto library, iTunes library, Adobe CS files, and so on. This will make your backup routine much easier. I even keep frequently used sample libraries on this volume.

Volume 4 : I also like to have one more partition for different OS version. You just bought Leopard, but you don't feel it is stable enough for daily jobs yet. You install Leopard on the separate partition so you can enjoy the new OS while you can go back to previous OS for jobs.

The benefit goes like this. Even after you completely moved to the new OS, the old OS volume stays as emergency boot volume. This has saved my life a few times. By the time another OS version is released, the current OS volume becomes the backup. You reformat the previous OS volume and install the spanking new OS for test drive until it becomes stable enough for your work.

Volume 5 : If you do video editing, you probably need a dedicated external hard drive. But if you just want to do a small iMovie for your short family movies, a separate partition can do the job. I have been doing my demo movie clips on a dedicated partition.