Towers are upgradeable. Even the Cube is upgradeable. Laptops are upgradeable. A mini is a mini and would loose its form factor if it had extra slots and such. It would go go from a sandwich to a toaster. There are sites that show you how to hack OS X onto a Pee Cee. But at $500.00 the mini is no more upgradeable than a PDA or Treo.

There are upgrade cards http://eshop.macsales.com/shop/accelerators/