According to a utility that came with my ORB drive, my Mac
has two internal IDE buses (which are used for the hard disk and the CD-ROM),
However, according to
this document,
whether or not it can make sense of more than one device on each bus depends
on whether the logic board revision is 2 or newer.
Apple System Profiler says nothing about logic board revisions, but also
says nothing about ATI RAGE chipsets, which suggests that it may be one of
the older ones. Which means that bolting on a nice big hard disk may be
easier said than done...