Hi,
Since OSX installs and runs only on apple hardware, you should get the OSX dvd image from the osX86 project, they patched the kernel to boot on any x86 machine.
Second i have never tried it before, but i think it will overwrite your bootloader, but fear not! you can simply reinstall GRUB with your ubuntu live cd.
P.S. Dont forget to check your hardware compatibility with the OSX
.