Hideo Takano has perfected the art of the Mac Mini mod. Fitting the guts of Mac Mini's inside a miniaturized Mac Pro, he's created the Mac Mini Pro.
Inside, he's housed a powerful Mac Mini with a 2.33GHz Intel Core 2 Duo processor and 2GB of RAM. His imitation of the Pro goes so far as to house 500 GB hard drive in the same location as it's big brother. If you don't like the standard look of the Mac Mini, why not follow in his footsteps and build your own?
Check out his site, Mac Mod Lab, for this and many more Mac mods. Takano is pretty talented!