SQL / per cent change question

Howdy:

Currently running PostgreSQL 7.1.3 on RedHat Linux.

What I'm trying to do is create a table where I have
data loaded every week or so. In that table I'll have
simple things like name, count, date, whatever.

What I'm looking for is for a method to do per cent
difference from each last count that was loaded into
the table.

For instance, I load something like this:

insert into t_change (name, count, date
values ("widgets", "2304", timestamp(now()))

That give me my table and some info, right? Now,
every time that someone updates that table, I've seen
a formula on how to get the per cent between
two columns already inserted in the table, but what
can I do when I have only ONE field and want to get the
difference between that and the load PRIOR to
what I have? That is to say, how can I get the
difference / percent change between "count" and
the count that was loaded, say, a few minutes before
on a routine basis?

Suggestion? Comments? Thanks!

-X