cost and actual time

hello all,

I am still fiddling around with my "big" database.

System:
RAM: 2GB
CPU: 1,6 MHz (cache: 256 Kb)
single disc: 120 GB :-(

I have a query that joins to relatively large tables (10 - 15 Mio rows),
or part of these tables (explain analyze: rows=46849) respectively.

long story short:

allover cost estimated in pages by explain is:
cost=6926.59..6926.60

actual time is from explain analyze is:
actual time=275461.91..275462.44

most of it is consumed by a nested loop (surprise!)
this is the respective output:

Sort Key: disease.disease_name, disease_occurrences.sentence_id
->  Nested Loop  (cost=0.00..6922.38 rows=98 width=64) (actual
time=61.49..275047.46 rows=18910 loops=1)
   ->  Nested Loop  (cost=0.00..6333.23 rows=98 width=28) (actual
time=61.42..274313.87 rows=18910 loops=1)
     ->  Nested Loop  (cost=0.00..5894.04 rows=64 width=16) (actual
time=32.00..120617.26 rows=46849 loops=1)

I tried to tweak the conf settings, but I think I already reached quite
a good value concerning shared buffers and sort mem. the database is
vacuum full analyzed. indexes seem fine.

could one of you smart guys point me into a direction I might not have
considered? - I know that the hardware is the minimum. nevertheless - if
you have suggestions what exactely to add to the hardware to boost the
database up (more RAM or more discs - even a RAID) - this would be a
good argument for my boss.

Thank you a lot
Chantal