the system cards are a combination of RAM and a driver for the CDROM. without a system card, you cannot play cd games because the system doesn't know it exists. there are several variations of the system card, but the 4 you need to know about:
1.0; the first one, japan only. some games will only work with this one.
2.0; the one americans got with the cd attachment.
super system; included inside the duo. requires a separate card if you have the old combo.
arcade system. the last one. japan only. comes in two flavors, pro and duo. the duo version only works with duos; the pro works with anything (i think.)