CASE Select, referring to selected value

Hi!
I've got 2 date fields in table which constitute a term.  On select I'm
calculating that term, BUT I can't figure out how to refer to it in the
sql - or if it's possible.  I'd like to be able to use the case returned
value for order, comparing, etc.  Here's an example:

select case when (current_date > available) then ((end_date -
current_date) / 30)            when (current_date < available) then ((end_date - available) /
30)      end      from listing

This yields something along the line of:
case
------   2  39  11  64
...

which is great.  Now, I'd like to be able to do this for example:

select case when (current_date > available) then ((end_date -
current_date) / 30)            when (current_date < available) then ((end_date - available) /
30)      end      from listing      where case > 4

which yields:
ERROR:  parser: parse error at or near ">"

Does anyone know how I could accomplish this?

Thanks in Advance!
Nick