Application requirements gathering: tools

["Reeve Fritchman" <reeve@xxxxxxxxxx>]

I'm in the middle of gathering requirements for a couple of very large
projects and I'm trying to find a tool I can use to organize factoids
into one or more functional areas.  In the transportation/logistics
arena, the links between various application areas are numerous and
tight.  
 
I'd like to be able to enter something like, "If 'x', then 'y' and 'z'."
Subsequent processing/diagrams/output would show ''x" as a parent or
predecessor function to 'y' and 'z'; 'y' and 'z' would should
dependencies on 'x'.  Then, when working on detailed design for 'x',
'y', or 'z', I'd be able to review the complete set of links.  I'm
referring process and functional requirements, not to column/table
dependencies.
 
Even a small project, when fully defined, can generate a couple of
hundred factoids.  Having this type of "knowledge base" for an
application could cut way down on the number of "gotcha"'s that tend to
arise when making anything other than minor application changes.
 
Has anybody found a tool like this, or do I have to buy a couple of
cases of Post-It's?
 
Thanks,
Reeve