out of memory during query execution

Hello,

I am a PostGreSQL newbie. I work with the 8.1.0 release on AIX 5.3, with
300GB of datas.
Some of the queries launched on this database finish with an "*out of
memory*". The queries which have failed contain a lot of join (between 6
tables), sub-select and aggregate. For these queries, the log file
contains :
psql:Q9.sql:40: ERROR: out of memory
DETAIL: Failed on request of size 148.

On the server used, I got 3GB of memory and 1 CPU.
The settings specified in the "postgresql.conf" are :
# - Memory -
shared_buffers = 12288
#temp_buffers = 1000
#max_prepared_transactions = 5
work_mem = 65536
maintenance_work_mem = 262144
max_stack_depth = 24574

Are some of these values false?
Is the "out of memory" error due to smaller memory available ?
Has somenone ever seen this problem ?

Thank you for your help.

Regards,
Alexandra DANTE