dual boot hoary-macosx

[Eduardo Sáenz de Cabezón]

hi, 
i have installed ubuntu /hoary) on an imac G3, I would like to know how
to configure the system to choose wether to boot ubuntu or macosx. My
yaboot.conf file is the following:

## yaboot.conf generated by the Ubuntu installer
##
## run: "man yaboot.conf" for details. Do not make changes until you
have!!
## see also: /usr/share/doc/yaboot/examples for example configurations.
##
## For a dual-boot menu, add one or more of:
## bsd=/dev/hdaX, macos=/dev/hdaY, macosx=/dev/hdaZ

boot=/dev/hda12
device=/pci at f2000000/mac-io at 17/ata-4 at 1f000/disk at 0:
partition=13
root=/dev/hda13
timeout=50
install=/usr/lib/yaboot/yaboot
magicboot=/usr/lib/yaboot/ofboot
enablecdboot

image=/boot/vmlinux
	label=Linux
	read-only
	initrd=/boot/initrd.img
	append="quiet splash"

image=/boot/vmlinux.old
	label=old
	read-only
	initrd=/boot/initrd.img.old
	append="quiet splash"


I would like to know what should I exactly add to this file, how to know
which hdaZ to add, if I have to do something else...

Thanks for your help