I only have a little experience with Netbooting, but it sounds like you might be trying to boot a machine from a netboot image it can't run properly.
That is, the "hanging" message is (I assume) a kernel panic, which (since it's that early in the boot process) is probably caused by a low level incompatibility. Since the one that works is a G5 and completely different model, I'm guessing maybe you're trying to use the same boot image but it isn't compatible with the powerbook.