Last things first.

Rio was sold to another company who is continuing to fund production of mp3 players. The empeg shares a codebase with them and one of the developers has said that we'll be getting used as unofficial software testers for new features throughout the line.

In fact, the 2.0 final software was released before Rio went under, IIRC. (Although it does have a few bugs left in it.)

Otherwise, any 40GB drive should do fine as long as it is within the height limitation for MkI empegs, which is 14mm. You might want to put some washers between the drive tray and the drive itself so that the drive's spindle doesn't rest directly on the tray; this has been the cause of some problems in the past, especially with some IBM drives, as I believe it states in the Drive Upgrade Guide.

That being said, I'll leave you to investigate the dimensions of those drives.