Even the guide on the front page, which is quite detailed and handsome-looking, doesn't give all the information I need. A couple questions remain.
1: Do I need a System Card to play any games at all in a gray DUO or white CD-ROM2 unit, or are they built in?
2: Is the Arcade Card DUO essentially the ultimate system card? Will it allow me to play Super CD-ROM2 games as well as Arcade Card games?
1: The Super CD System Card is built in to All Duo systems(Turboduo, PCE Duo, Duo-R, 'RX). So a Duo system will play all CD games(worldwide) except Arcade Card CDs and all HuCards native to it's region.
The white CD-ROM2 unit requires a PC Engine system(white PCE, Coregrafx I or II, or Supergrafx or LT + RAU-30 adaptor)
and a System Card(which is played through the PCE system).
2: The Arcade Card Duo just lets a Duo system play Arcade Card games as well as everything else. To play all CD games on a white CD-ROM2 system, you'll need the Arcade Card PRO(and some kind of PCE system).
Please read the NEC Compatibility Guide that should answer your questions.
I think that that was what he was refering to by "the guide on the front page".