Sticking a Mini into an older Mac is a natural way to modernize it. A Cube is easy, or a 128K, or an Apple II, if you want to run Lion on one. The biggest obstacle is often the monitor. OTOH, it means sacrificing the old computer as well as a new Mini which is kinda cool in its own right. As for Cubes, they are cheap and plentiful on eBay, especially as-is examples without power supplies, so that probably wouldn't stop me. As it happens I have a spare cube chassis which is a good candidate, but I don't have a spare Mini, and new Minis aren't all that cheap.

I did make one of my Minis into a miniature aluminum Cube containing a 3.5" HD and USB & FW hubs. It's basically just an aluminum wrap-around shell for the Mini and an OWC MiniStack. A Mini-skirt...

http://web.me.com/henryspragens/stuff/Photos/Pages/Computers.html#5