In my mind, "put Ubuntu on a Power Mac G4" isn't really very exciting. It would have qualified for the first RetroChallenge, but it seems like
RC has become more project-based. The entries I've seen over the past few years have involved restorations, learning how to do a certain task, or programming things. The person who hosts it does a lot of things with DEC systems, too, but I also see a lot of stuff being done with 8 and 16-bit microcomputers.
The RetroChallenge rules now explicitly say that a computer that "can run Windows XP" is probably too new. A lot of
really old things from 1996 will happily run Windows XP, so I think that gives you an idea of what they're expecting. They say exceptions can be made for things that are exotic, but I'm thinking that that would involve something like "building a
Briel repica 1!" or doing a project on something that almost nobody has in their home.
I personally wouldn't count anything NewWorld at all, on the Mac side of things.
It can be interesting to match old hardware with new software (or the reverse!) but it can be a little anti-climactic. The whole point of the RetroChallenge was to push boundaries and get people excited about older computers. (It was probably one of the first things 68kMLA did that acknowledged 68k Macs as something that might not be totally practical as every-day main computers any longer.)
Ultimately, if your needs are the same today as they were ten to fifteen years ago, then that's fine, but when I see this kind of thread where somebody wants to use a PowerPC Mac for any length of time as a "challenge" all I can think is what if I did the same thing with my Microsoft Surface RT (2012, but really slow), or my ThinkPad X31 (2003). I ruminate on it for a moment and then realize that it would be like my daily life.
That said, as IPalindromeI alluded to, the 68kMLA is really focused more on 68k Macs. We expanded a few years ago, and to be honest, I would personally like to contract back again -- possibly limit to beige/oldworld PowerPCs and Mac OS 9. (Due to connectivity, Mac OS 9 might actually be an interesting productivity challenge, but it depends on how/where/with-whom you work, and one of the original stipulations of the RetoChallenge Endurance "Competition" was that you could use work-provided resources for work purposes without an impact on your score.)
And add to that my entire 2 and a half week long music library is stored on my hard drive, and while this would be theoretically possible on the 9600, the 36Gb cap on SCSI hard disks would make it impractical to store all my music locally,
There are SATA cards that'll happily run the biggest SATA disks you can find in beige PowerPCs. If you use Mac OS 8.1 or newer you can use HFS+ to make very large volumes efficiently.
SCSI itself also isn't limited to 36 gigabytes, but SCSI disks bigger that can be somewhat difficult to find, as most of them are probably being snapped up by people with systems where a SATA or IDE card isn't a practical choice.