A Cinco De Mayo update, in case anyone is/was interested:
Debian had to go, booo. I couldn't take the dependency breaks of Testing, and Stable, while probably great in the long term, left me desiring for more current software. It turns out the earlier bootloader problems came back to haunt me, this time for good.
Despite of an out-of-sync master boot record, of which I learned about only later on (Apple uses a hybrid MBR, ugh), I guess with just pure luck I was able to install Debian the first (second?) time, but now, after wiping it all clean and going back to Ubuntu (and Fedora, and openSUSE, and Arch again, and Ubuntu again, rinse and repeat...), I was stuck in a loop. OS would install, but a boot would always result in the same dreaded
grub rescue> -screen, with no way to recover. If, against all odds, I'd get a successful yet random boot, the GPU drivers would not work, and I would start from scratch. The issue with the MBR could be resynced with rEFIt I had already installed on OSX, except it was missing the crucial 'gptsync' option that would've saved the day, supposedly. Or, I could go for the more up-to-date rEFInd bootloader with all kinds of boot and sync options, but requiring a bit more setting up.
More googling, mainly from Rod Smith's excellent Wiki-level web pages, rEFInd would work better. But in order to install rEFInd on Mac's El Capitan, unlike earlier versions of OSX, I'd have to boot into the Recovery Partition of OSX, and disable Apple's built-in 'System Integrity Protection' -- a safeguard against users like me who think they know what they are doing, yet... Turns out, my Recovery HD was corrupted leading to constant and consistent kernel panics (probably the culprit of MBR issues as well), so in the end, this wasn't going to work. If no Recovery --> no disabling SIP --> no rEFInd --> no resyncing of MBR/GPT --> no Linux. Bleh. End day, open box wine.
After a glass or two, decision was made. OSX was going to be launched into the sun. Via gdisk I'd wipe, clean and zap all drives and subsequent MBRs. Onward with Linux! Again.
The nagging GPU driver issue was still in the back of my mind though. Turns out, again, user error, again.
Some, if not most, somewhat recent Nvidia cards do work in these early 2006-2007 Mac Pros, even when they are not officially supported, or specifically flashed for Macs (another can of worms if you want to flash the GPU BIOS to be more Mac compliant). The downside of running an Nvidia card on an old Mac is that unlike with a supported AMD GPU, Mac passes on graphics duties to the Nvidia card very late in the boot process. So every time I'd have to access boot options, for example to boot from DVD, I'd have to have a monitor plugged in to an old Mac AMD GPU, and only after crap was sorted out, would I reboot with the the Nvidia plugged in. To minimize PITA, I started leaving the Nvidia card in, and just plugged in and out of the AMD card when needed. And therein lied the problem, or so I deduced, with said Nvidia drivers. I proved to myself the hard way that having two different GPU's plugged in during OS and/or driver installation gives you only about a 7.5% success rate Almost Vegas odds.
But all that begins well, must end well. After all the swapping, wiping, cleaning, cursing, and box wine, Ubuntu 17.04 is it. And it's on. With latest Nvidia drivers, Steam running and all the other latest-and-greatest I've come to enjoy with Linux, and while Gnome is my homeboi, I'm liking Unity for the time being. And after all this, I'm pretty sure I could get Debian Testing running again if I wanted, or even Arch? Maybe I should, just to prove a point... Do I have any of that box wine left?