I would not disagree with Delta.. I could have sworn the most late G4s and all G5 were supposed to be USB bootable. I do know that about a year ago I had to rescue the same vintage G5 and an early Intel Powermac and I did have trouble booting from USB on the former. I ended up creating a FW boot volume for the job partitioning one of my FW Hard Drives into a 8GB Partition and a Regular size one and I ran the installs from the HD.
You can use Disk Utility to restore the 10.5 Disk Image of the DVD from your Macbook pro to the 8GB partition. Macsales sells good ones, if you do not have a FW drive based system. When you do firewire its best to use something oxford chipset based or you can end up in heck. They start at around $100. There are options too if you already have drives.
We can not divine what your using the old G5 for but in general I would put money towards things that would be reusable like the multi-interface case from macsales. Nothing wrong with fixing up an old machine if there is a reason. Its true that a Sata inerface card wold let you use new things on the inside. I did see FW burners but there pricy.