"Out of memory" errors..

Hi

I am getting the following error while running queries such as "vacuum
analyze TABLE", even on small tables with a piddly 35,000 rows!

The error message:
--
ERROR: out of memory
DETAIL: Failed on request of size 67108860.
--

My postgresql.conf is below. I am on a Dual Core server with 4GB or
RAM, which runs MySQL as well (key_buffer for which is at around
800M). So I have allocated shared_buffers for postgresql based on that
number. The server also runs Apache and other stuff, but I have never
had any problem running the vacuum equivalent called "REPAIR TABLE" on
MySQL.

Thanks in advance for any inputs!

------POSTGRESQL.CONF-------
#--- Some tuning ~
#--- http://www.opennms.org/index.php/Performance_tuning
max_connections = 250
shared_buffers = 21000
effective_cache_size = 21000
max_fsm_relations = 1500
max_fsm_pages = 80000
sort_mem = 16348
work_mem = 16348
vacuum_mem = 16348
temp_buffers = 4096
authentication_timeout = 10s
ssl = off

autovacuum = on
vacuum_cost_delay = 50
stats_start_collector = on
stats_row_level = on

#--- For COPY performance
wal_buffers=64
checkpoint_segments=64
checkpoint_timeout=900
fsync = on
maintenance_work_mem = 64MB