You can't build your own guts.  The closest you could get to building your own is putting the guts (mobo, cpu etc.) of a powermac in a different case.  you could buy a mobo with cpu and psu on ebay and throw it into an ATX case.

Hope this helps..
Mac mini C2D 1.83GHz - 2GB RAM - 200GB 7200rpm HD
Sawtooth w/G4 1GHz - 1GB RAM - SIIG SATA w/2x1TB HD
B&W w/G4 600MHz - 1GB RAM - 2x80GB HD
Sawtooth G4 400MHz - 512MB RAM - 40GB HD
Dell Ultrasharp 2007WFP 20" Widescreen LCD
2.94TB Total Storage