Planner very slow on same query to slightly different tables

I'm using Postgres 7.2.1 on a dual-Athlon running RedHat 7.3bigmem
with 2 Gig of RAM and a 240 Gig RAID 5 (3ware IDE RAID). I just did a
'vacuum analyze' on the database, however the same query to two
similar tables is coming up quite different. The two tables only
differ in that one ("center_out_cell") has an extra int2 field called
"target" which can take up to 8 different values.

Here are the queries:

db02=# explain select distinct area from center_out_cell where subject
= 'M' and arm = 'R' and rep = 10 and success = 1 and direction = 1;
NOTICE:  QUERY PLAN:

Unique  (cost=87795.47..87795.80 rows=13 width=5) ->  Sort  (cost=87795.47..87795.47 rows=131 width=5)       ->  Seq
Scanon center_out_cell  (cost=0.00..87790.87 rows=131
 
width=5)

EXPLAIN
db02=# explain select distinct area from circles_cell where subject =
'M' and arm = 'R' and rep = 10 and success = 1 and direction = 1;
NOTICE:  QUERY PLAN:

Unique  (cost=258.36..258.52 rows=6 width=5) ->  Sort  (cost=258.36..258.36 rows=64 width=5)       ->  Index Scan using
pk1circles_cellon circles_cell 
 
(cost=0.00..256.43 rows=64 width=5)

EXPLAIN


Here are the definitions for the 2 tables:

db02=# \d center_out_cell          Table "center_out_cell"  Column   |        Type        | Modifiers
------------+--------------------+-----------subject    | text               |arm        | character(1)       |target
 | smallint           |rep        | integer            |direction  | smallint           |success    | smallint
|hemisphere | character(1)       |area       | text               |filenumber | integer            |dsp_chan   | text
           |num_spikes | integer            |spike_data | double precision[] |
 
Unique keys: pk0center_out_cell,            pk1center_out_cell

where: 
db02=# \d pk1center_out_cell
Index "pk1center_out_cell"  Column   |     Type
------------+--------------subject    | textarm        | character(1)target     | smallintrep        |
integerhemisphere| character(1)area       | textfilenumber | integerdsp_chan   | textdirection  | smallint
 
unique btree
Index predicate: (success = 1)


and
db02=# \d pk0center_out_cell
Index "pk0center_out_cell"  Column   |     Type
------------+--------------subject    | textarm        | character(1)target     | smallintrep        |
integerhemisphere| character(1)area       | textfilenumber | integerdsp_chan   | textdirection  | smallint
 
unique btree
Index predicate: (success = 0)


db02=# \d circles_cell           Table "circles_cell"  Column   |        Type        | Modifiers
------------+--------------------+-----------subject    | text               |arm        | character(1)       |rep
 | integer            |direction  | smallint           |success    | smallint           |hemisphere | character(1)
|area       | text               |filenumber | integer            |dsp_chan   | text               |num_spikes |
integer           |spike_data | double precision[] |
 
Unique keys: pk0circles_cell,            pk1circles_cell

where:

db02=# \d pk1circles_cell Index "pk1circles_cell"  Column   |     Type
------------+--------------subject    | textarm        | character(1)rep        | integerdirection  |
smallinthemisphere| character(1)area       | textfilenumber | integerdsp_chan   | text
 
unique btree
Index predicate: (success = 1)

db02=# \d pk0circles_cell Index "pk0circles_cell"  Column   |     Type
------------+--------------subject    | textarm        | character(1)rep        | integerdirection  |
smallinthemisphere| character(1)area       | textfilenumber | integerdsp_chan   | text
 
unique btree
Index predicate: (success = 0)


Now I know that, due to the extra field "target", "center_out_cell"
can be as large as 8 times "circles_cell", but according to the cost
of the planner, the statement is 340 times more costly.  I think this
is because the planner is using the index in the circles_cell case and
not in the center_out_cell case. However, I don't pretend to
understand the intricasies of the planner to make an intelligent
guess. I've been trying random changes to postgresql.conf like
increasing the shared memory size, changing the random_page_cost size,
etc., but would like some help in trying to speed things up.

Here are some relevant settings from my postgresql.conf (made in an
attempt to max out buffers):

shared_buffers = 9000      # 2*max_connections, min 16 
wal_buffers = 32            # min 4
sort_mem = 64000         # min 32 
vacuum_mem = 16384          # min 1024
wal_files = 32
effective_cache_size = 1000  # default in 8k pages


Thanks in advance.
-Tony