BootX is what you use for Old World PPC Macs (Macs that still have a hardware ROM in them). Yaboot is what you use for New World Macs (basically, any PPC Mac that uses USB since those were the first with a usable version of OpenFirmware).
Incidentally, there are some older Macs and Mac clones like the StarMax 4000 which are considered Old World Macs even though they do have an early version of OpenFirmware in them. However, the OpenFirmware in these particular Macs was so buggy that people trying to use yaboot would end up bricking their Macs/clones. What's funny is that this particular early version of OpenFirmware was only visible from the serial port if you were connecting through there using a terminal emulation console (like those used years ago to connect to BBSes using a modem).
Any information about Linux/ppc and their bootloaders can be found here:
http://www.penguinppc.org
Incidentally, there are some older Macs and Mac clones like the StarMax 4000 which are considered Old World Macs even though they do have an early version of OpenFirmware in them. However, the OpenFirmware in these particular Macs was so buggy that people trying to use yaboot would end up bricking their Macs/clones. What's funny is that this particular early version of OpenFirmware was only visible from the serial port if you were connecting through there using a terminal emulation console (like those used years ago to connect to BBSes using a modem).
Any information about Linux/ppc and their bootloaders can be found here:
http://www.penguinppc.org