The Cube was absolutely the most beautiful computer Apple ever made, except for maybe the 20th Anneversary Mac.
If you have the cash, you can upgrade one to a 1.25Ghz G4 (or maybe a dual, I'm not sure), and you can also put most new graphcs cards in one, but some have to be shaved a bit (!) to get them to fit.
Again, they're beautiful, but a bit impractical for upgrades.
A Cube would make a great headless server tucked away in a small corner too.