I second the external drive option if you don't need a drive in every machine. The reason I suggest this is that getting to the optical drive in eMacs can be a pain in the tuchkus. A royal pain. Exposing CRT's can be dangerous if you don't know what you're doing, and the sheer number of screws is similar to those in a hard drive replacement for an iBook G4.
But hey, if you're installing the optical drives, it gets easier every time and regular drives fit right in.