JOIN work somehow strange on simple query

Hi. I have a performance problem with this simple query:

SELECT collect_time FROM n_traffic JOIN n_logins USING (login_id)
WHERE n_logins.account_id = '1655' ORDER BY collect_time LIMIT 1;
----------------------------------------
Limit (cost=0.00..2026.57 rows=1 width=8) (actual
time=5828.681..5828.681 rows=0 loops=1)
-> Nested Loop (cost=0.00..901822.69 rows=445 width=8) (actual
time=5828.660..5828.660 rows=0 loops=1)
-> Index Scan using n_traffic_collect_time_login_id on
n_traffic (cost=0.00..10280.58 rows=281608 width=12) (actual
time=0.026..1080.405 rows=281655 loops=1)
-> Index Scan using n_logins_pkey on n_logins
(cost=0.00..3.15 rows=1 width=4) (actual time=0.011..0.011 rows=0
loops=281655)
Index Cond: ("outer".login_id = n_logins.login_id)
Filter: (account_id = 1655)
Total runtime: 5828.963 ms
(7 rows)

This looks very argly... I done some hack and change the
JOIN n_logins USING (login_id) WHERE n_logins.account_id = '1655'
with the
WHERE (login_id = '1' OR login_id = '2'...)
in script which prepare this ...OR...OR... list and form this query.
But THIS IS DIRTY HACK! There must be gooder way... Please, help,
explain!

If there is only one login_id сorresponds to some account_id the query
goes fast:

=# explain analyze SELECT collect_time FROM n_traffic JOIN n_logins
USING (login_id) WHERE n_logins.account_id= '15' ORDER BY collect_time
LIMIT 1;
---------------------------------------------------------
Limit (cost=1262.93..1262.94 rows=1 width=8) (actual
time=61.884..61.886 rows=1 loops=1)
-> Sort (cost=1262.93..1263.49 rows=223 width=8) (actual
time=61.867..61.867 rows=1 loops=1)
Sort Key: n_traffic.collect_time
-> Nested Loop (cost=5.60..1254.23 rows=223 width=8)
(actual time=3.657..36.890 rows=4536 loops=1)
-> Index Scan using n_logins_account_id on n_logins
(cost=0.00..3.22 rows=1 width=4) (actual time=0.032..0.049 rows=1
loops=1)
Index Cond: (account_id = 15)
-> Bitmap Heap Scan on n_traffic (cost=5.60..1241.72
rows=743 width=12) (actual time=3.601..19.012 rows=4536 loops=1)
Recheck Cond: (n_traffic.login_id = "outer".login_id)
-> Bitmap Index Scan on n_traffic_login_id
(cost=0.00..5.60 rows=743 width=0) (actual time=3.129..3.129 rows=4536
loops=1)
Index Cond: (n_traffic.login_id = "outer".login_id)
Total runtime: 63.697 ms
(11 rows)

Tables:

=# \d n_traffic
Table "public.n_traffic"
Column | Type | Modifiers
--------------+-----------------------------+------------------------------
login_id | integer | not null
traftype_id | integer | not null
collect_time | timestamp without time zone | not null default now()
bytes_in | bigint | not null default (0)::bigint
bytes_out | bigint | not null default (0)::bigint
Indexes:
"n_traffic_login_id_key" UNIQUE, btree (login_id, traftype_id, collect_time)
"n_traffic_collect_time" btree (collect_time)
"n_traffic_collect_time_login_id" btree (collect_time, login_id)
"n_traffic_login_id" btree (login_id)
"n_traffic_login_id_collect_time" btree (login_id, collect_time)
Foreign-key constraints:
"n_traffic_login_id_fkey" FOREIGN KEY (login_id) REFERENCES
n_logins(login_id) ON UPDATE CASCADE
"n_traffic_traftype_id_fkey" FOREIGN KEY (traftype_id) REFERENCES
n_traftypes(traftype_id) ON UPDATE CASCADE

=# \d n_logins
Table "public.n_logins"
Column | Type | Modifiers
------------+------------------------+---------------------------------------------------
login_id | integer | not null default
nextval('n_logins_login_id_seq'::regclass)
account_id | integer | not null
login | character varying(255) | not null
pwd | character varying(128) |
Indexes:
"n_logins_pkey" PRIMARY KEY, btree (login_id)
"n_logins_login_key" UNIQUE, btree ("login")
"n_logins_account_id" btree (account_id)
Foreign-key constraints:
"n_logins_account_id_fkey" FOREIGN KEY (account_id) REFERENCES
n_accounts(account_id)
Triggers:
tr_after_n_logins AFTER INSERT OR DELETE OR UPDATE ON n_logins FOR
EACH ROW EXECUTE PROCEDURE tr_f_after_n_logins()
tr_before_n_logins BEFORE UPDATE ON n_logins FOR EACH ROW EXECUTE
PROCEDURE tr_f_before_n_logins()

My machine is 2xPIII 1 GHz + 1 Gb RAM + RAID5 on 6 SCSI disks. My settings is:
max_connections = 50
shared_buffers = 32768
temp_buffers = 57792
work_mem = 81920
maintenance_work_mem = 196608
max_fsm_pages = 262144
max_fsm_relations = 1000
wal_buffers = 64
checkpoint_segments = 4
checkpoint_timeout = 300
checkpoint_warning = 30
effective_cache_size = 6553
random_page_cost = 3
default_statistics_target = 800
log_rotation_age = 1440
log_line_prefix = '%t %u(at)%d '
stats_start_collector = on
stats_command_string = on
stats_block_level = on
stats_row_level = on
stats_reset_on_server_start = on
---
# ipcs -T
msginfo:
msgmax: 16384 (max characters in a message)
msgmni: 40 (# of message queues)
msgmnb: 2048 (max characters in a message queue)
msgtql: 40 (max # of messages in system)
msgssz: 8 (size of a message segment)
msgseg: 2048 (# of message segments in system)

shminfo:
shmmax: 838860800 (max shared memory segment size)
shmmin: 1 (min shared memory segment size)
shmmni: 128 (max number of shared memory identifiers)
shmseg: 128 (max shared memory segments per process)
shmall: 204800 (max amount of shared memory in pages)

seminfo:
semmni: 256 (# of semaphore identifiers)
semmns: 2048 (# of semaphores in system)
semmnu: 256 (# of undo structures in system)
semmsl: 60 (max # of semaphores per id)
semopm: 100 (max # of operations per semop call)
semume: 10 (max # of undo entries per process)
semusz: 100 (size in bytes of undo structure)
semvmx: 32767 (semaphore maximum value)
semaem: 16384 (adjust on exit max value)
---
kern.seminfo.semmni=256
kern.seminfo.semmns=2048
kern.seminfo.semmnu=256
---
postgresql have its own login class:

postgresql:\
:openfiles-cur=768:\
:tc=daemon:
daemon:\
:ignorenologin:\
:datasize=infinity:\
:maxproc=infinity:\
:openfiles-cur=128:\
:stacksize-cur=8M:\
:localcipher=blowfish,8:\
:tc=default:
--
engineer