I have been looking around to find information reading the different System card versions and their differences. I have compiled the following list from several sources (listed at the end!) and thought that others may find this useful.
Please feel free to comment and correct any detail.
Accessing saved data management menu:To access the save data management menu press "Select" on the splash screen. The following options are shown on a menu (top to bottom) Delete, Format, End
System Card 1.0 * Bundled with the debut Japanese CD-ROM system release (Not released in the US)
* Also available for purchase
* Lacks CD+G support,
* Only card that will allow the CD version of Altered Beast to play flawlessly
* Has built-in memory viewer and debugger (Hold Up + Right + Button I + Button II, then press Select)
System Card 2.0* Bundled with the debut US CD-ROM system release (Also released in Japan)
* US Version start up screen shows "TurboGrafx CD 2.0"
* Adds CD+G support
* Debug function that came with 1.0 removed
* 1 MegaBit of RAM
System Card 2.1* Not packaged with console release, sold separately
* Added an auto disc-change feature
* 1 MegaBit of RAM
System Card 3.0 (Super System Card)* Not packaged with console release, sold separately
* Released in Japan and the US
* Support for Super CD games
* 2 Megabits of RAM
Arcade Card Duo (For DUO/DUO-R/DUR-RX/SUPER CD-ROM)* Not packaged with console release, sold separately
* Support for Arcade CD releases (Strider, Neo-Geo conversions, Sapphire, etc)
* Does not contain System Card 3.0 BIOS as this is already built into the Duo / super systems
* Start up screen displays "Super CD-ROM2 version 3.00"
* 16 Megabits of RAM (16 Mebabits on card + 2 Megbits internal Ram = 18 Megabits)
Arcade Card Pro (For original PC engine with CD-ROM attachment)* Not packaged with console release, sold separately
* Support for Arcade CD releases (Strider, Neo-Geo conversions, Sapphire, etc)
* System Card 3.0 BIOS on card
* Start up screen displays "Super CD-ROM2 version 3.00"
* If you use an Arcade Card Pro card with a US Duo you will get you the PCE SuperCD screen
* 17.5 MegaBits of RAM
Game Express card 1.0* Used to play of unlicensed Game Express CD games
* CD Express Games do not work with official system cards
* Can only access 64k of memory that is built into the CD unit
* Can only play earlier Game Express titles
Game Express card 1.1* Used to play of unlicensed Game Express CD games
* CD Express Games do not work with official system cards
* Can access the 192k+64k of memory in the way that the arcade card DUO does, that's on the Super CDROM 3.0 unit and the Duo models.
* Can play all Game Express titles
Credit where due:The following web resources were used in the creation of this version list.
http://www.lingjr.com/collection_nec_pce.htmhttp://magweasel.com/2009/07/17/i-love-the-pc-engine-cd-rom-system/Also thanks to the following members of
https://www.pcenginefx.com/forumsBonknuts: Game Express card versions and technical detail
Necromancer: Debugger access method for system card version 1.0 (Via
http://www.pcecp.com/)
Td741: Arcade Card Pro card with a US Duo info