For selecting a new HDD, it doesn't matter which MDD G4 you own. Their capabilities and limitations regarding HDDs internal to the case are exactly the same. However, if you want to add an external HDD via Firewire, then the FW800 MDD G4 offers you one more option. Look on the back of the case for a 3rd Firewire connector that is shaped differently than the other two Firewire connectors. If you only have 2 Firewire connectors, you don't have a FW800 MDD G4.
Carl B.