low memory usage reported by 'top' indicates poor tuning?

Hello,

I'm trying to make sense of the memory usage reported by 'top', compared
to what "pg_database_size" shows.   Here's one result:

select pg_size_pretty(pg_database_size('production'));
 pg_size_pretty
----------------
 6573 MB

Now, looking at memory use with "top", there is a lot memory that isn't
being used on the system:

 Mem: 470M Active, 2064M Inact

( 3 Gigs RAM, total ).

Overall performance is decent, so maybe there's no
problem. However, I wonder if we've under-allocated memory to
PostgreSQL. (This is a dedicated FreeBSD DB server).

Some memory settings include:

shared_buffers = 8192 (we have 450 connections)
max_fsm_pages = 1250000 (we kept getting HINTs to bump it, so we did)

Maybe we should be bumping up the "sort_mem" and "vacuum_mem" as well?

I do sometimes see sorting and vacuuming as showing up as things I'd
like to run faster.

This list has been a great resource for performance tuning help, and I
continue to appreciate your help. We've used PostgreSQL on every project
we've had a choice on for the last 10 years. (Has it been that long?!)
We've never regretted it once.

   Mark