The defining line of NewWorld and OldWorld is ROM-in-RAM. That's it. The easiest way to tell the difference form an external hardware point of view is the all NewWorld machines have factory USB ports. It's that simple. iMac, B&W, Lombard and up are all NewWorld. All lesser PPCs that are not NuBus are OldWorld. The difference between NuBus and OldWorld is the use of PCI slots and OpenFirmware. All PCI Macs have OpenFirmware, of various versions.

There are two BootX boot loaders, so don't be confused by them. The first is the Linux boot loader that loads the Linux kernel from inside Mac OS. The second is the Apple OS X boot loader. Both are very different.