The si has a single expansion slot, that can be either PDS or Nubus, but not
both together. The card lies parallel to and above the motherboard & HD and
requires an adaptor slot to do this. There are Nubus and PDS adaptort cards.
Thus, what kind of slots you have depends on what kind of adapter card you
have. With the exception of the Radius rocket, all NuBus cards I know of work
in the si. PDS slots and thus cards are Mac specific, thus not all PDS cards
work in all Macs. 