For this we want to focus on development releases. In Debian, there's Testing and also Sid for advanced users. Ubuntu has development releases, too, but for this How-To we'll concentrate on Debian since Ubuntu is based on it anyway.
If you decide on Testing (Jessie as of this writing), you can install from a Wheezy disc and then upgrade, or you can directly install from a Jessie installer (found on this page). Debian Sid cannot be installed directly from a disc. You must either install Wheezy or Jessie first and then upgrade.
Installing Jessie directly is simple enough, so let's illustrate how to upgrade from Wheezy. Once any kind of Wheezy is installed, it could be just a base system without a GUI, you need to edit your /etc/apt/sources.list file. You'll see several download mirrors with either "stable" or "wheezy" in them. To upgrade to Testing, replace every instance of "stable" or "wheezy" with either "testing" or "jessie". If you want your system to stick with Jessie after it turns stable, use "jessie", but if you want to stay permanently on a Testing rolling release, use "testing".
Once you got all that worked out, save and exit and run the following command to update your repositories:
sudo aptitude update
Then to actually upgrade your system, run:
sudo aptitude full-upgrade
and your kernel and all packages will be upgraded to Testing.
The procedure to upgrade to Sid is basically the same, but Sid doesn't get any security updates or sid-updates, so comment out those lines (if you don't know what "comment out" means without googling, you probably shouldn't be running Sid). Then replace "wheezy" or "jessie", whichever you installed from, with "sid" or "unstable". Then run the two commands above and your computer will explode, I mean, you'll be all set.
From then on, you can install software and report any bugs you find with Debian's
reportbugprogram. It comes in both command line and GUI versions and includes onscreen instructions to walk you through the process. So say if you upgrade to Jessie and suddenly your sound won't work, you can report a bug against alsa-base and say "My system isn't loading the snd-powermac module. What the eff?" Or something like that.
So put those old 'Books and Power Macs to work and make those packages maintainers roll their eyes at looking up Big Endian/Little Endian problems.