I want to have the following:
Gen5 120GB(or higher)
Gen6 160GB(Or higher) (Gift for girlfriend)
Does anyone know what the details on ZIFs drives are? Am I able to put any ZIFs drive in and the iPod will run? Or do I have to go specific brands due to firmware lockdowns and the like?
From what I remember when I repair iPods, the iPod video (Gen 5) can take any standard 1.8" ZIF connector hard drives. There are people here on the forums that have had success with hard drives up to 240GB in size.
As for the iPod Classic, you should be able to upgrade the hard drive, but you have to bear in mind that the older "fat" iPod Classic 160GB models use a CE-ATA interface on their hard drives. But, AFAIK the 80GB, 120GB, and the newer 160GB models are all ZIF connectors, so I don't see any reason why they won't accept hard drive upgrades.
Like everything else, your mileage may vary. And as a side note, it's quite a tricky job getting the iPod Classics opened up; Apple made the tabs that attach the front face to the back metal, and they *CLAMP*!