An update!
Though the help of the resident guru
@eyoungren I've swapped out the stock Radeon 9000 Pro for a Radeon 9200, and I do believe the system is more or less in its final configuration. And, I say that because I'm currently posting this using Lubuntu 16.04 off a live disk, a thing that simply will not work with the stock Radeon 9000. I realize the difference the bus speeds between the PCI and AGP cards, the 9200 being a PCI video card, mean that in some tasks, the 9000 would run better, but since my goal was to also maintain OS 9 support, I don't think the any of that is really going to be an issue. If I really needed speed, I could always go after a Radeon 9800, but then I do already have a G5, so that's all moot.
That said, first impressions are that this is an improved setup. Everything just works, and I even get a standard DVI and VGA port, instead of that Apple Display Connector. Neat as it is, I don't exactly have a display for it. I do have an old Viewsonic CRT, however, and it does just fine.
When I'm done setting it all back up, this is going to be a fun little system, no doubt.