This leaves me with quite a few questions: What to do with Dora the old computer? What distro to put on Marigold the new computer? What desktop environment to put on Marigold? The first two have been answered: Dora will become a home server. I won't start tinkering with Dora until after moving next weekend (doing more than the absolutely basics now would be counterproductive since I'd have to do everything else again). As for distros, a new computer is a clean slate for playing with a completely new distro, and Arch Linux has been on my radar for awhile ever since I became a non-newbie at Linux. Now's my chance.
As for the third... that's the real question. Arch is known for being bleeding edge in its package offerings, meaning yes, it has the (ugly, in my opinion) GNOME 3 on it. I'm currently using GNOME 2 on Dora, and I've used KDE and Xfce on my laptop. Of the latter two, I like KDE more due to its massive customizability and yes, its prettiness. I still can't get around the fact that almost everything starts with K, and sometimes this makes things difficult to find. Luckily it appears that Arch gives you lots of flexibility in what to install, compared to other distros that bundle a bunch of software with the desktop environment. It may be a headache at first, but I'll be rewarded with lots of shiny.
This is what happens when you don't tell your kids about Linux. Someone else will. Parents, learn from this.