The "requirements" for Leopard are probably the loosest and most arbitrary in Apple's history of making OSs. I think they were set more with an eye toward performance than the minimum hardware that would actually run it.
A stock Leopard install will work on any system with a G4 processor, AGP graphics, and enough RAM(
@LightBulbFun did some torture testing in a VM and if I remember right I think he found the bare minimum of 92mb for the window server to load). How WELL it runs is another story, but you can certainly get it to run.
Things only get complicated when you have a computer with PCI graphics-I have an install lightbulbfun tweaked that will run fine B&Ws, Yikes!, and beige G3s(with G4s upgrades in the beige and B&W) although it runs into issues with some other systems.
@Intell has it running on an 8600. I have a 700mhz G4 in my 8600, but I haven't wanted to spend a few hours on Facetime with lightbulbfun(anyone who has done that will understand why
) trying to get it to boot. I also have a G4-upgraded Lombard, but I'm not convinced it's actually possible to run Leopard on it.
For AGP-based systems, all that's really required is to bypass the install checks. LeopardAssist is a great program that does just that, although I generally do a TDM install(all AGP-based G4s support TDM) or clone over an existing install. There's also getting hands-on with Open Firmware, which is what Leopard Assist fundamentally does with a nice GUI to make the process easy.
Whatever the case, a SATA card would definitely be a worthwhile upgrade and you will NOT find it bandwidth choked in a Digital Audio. I have one DA that is 100% SATA, and the difference is definitely noticeable. Even if you stick with spinners, you get the advantage of newer drives, which tend to be faster, quieter, and cooler.
Along those same lines, the thought of putting ATA drives in RAID 0 makes me uneasy. I have a Quicksilver that was set up that way from the factory and I've left it(it's a nice high end, BTO computer). The last ATA drives were made in 2012, though, and most of us are using drives of varying ages that are older than that. I WOULD NOT take my chances on mission-critical stuff in ATA RAID 0, and if I had to I'd be sure I backed up constantly(Time Machine is your friend). Aside from the previously mentioned QS, the only ATA drives I run in RAID are in my Xserve RAIDs. Those are 500gb enterprise-class drives that were vetted by Apple for use in that application(although that's not an absolute guarantee). They are in RAID 5 and I have NOS spares(factory sealed and pre-mounted to a sled) ready to go if I need them.
All of that aside, I would be hesitant these days to use a G4 for the sort of content creation being discussed. I do photo work on a dual 2.7 G5, and its showing its age when I process large files(from scans) with it. Handling 36mp RAW files grinds it to a halt. Even a first gen Mac Pro will run circles around a G5 when it comes to video files. It's not really even a matter of raw processing power either-that sort of stuff these days is just optimized for x86-64 and using a different architecture can be a handicap.
