Benchmarck PostgreSQL 8.1.4 MySQL 5.0.20 and Oracle 10g2

Hello,

I'm running a benchmark with theses 3 databases, and the first results
are not very good for PostgreSQL.

PostgreSQL is 20% less performance than MySQL (InnoDB tables)

My benchmark uses the same server for theses 3 databases :
Dell Power edge - Xeon 2.8 Ghz - 2 Go Ram - 3 SCSI disks - Debian
Sarge - Linux 2.6

The transactions are a random mix of request in read (select) and
write (insert, delete, update) on many tables about 100 000 to 15 000
000 rows.

Transactions are executed from 500 connections.

For the tunning of PostgreSQL i use official documentation and theses
web sites :

http://www.revsys.com/writings/postgresql-performance.html
http://www.varlena.com/GeneralBits/Tidbits/annotated_conf_e.html

Some important points of my postgresql.conf file :

max_connections = 510
shared_buffer = 16384
max_prepared_transactions = 510
work_mem = 1024
maintenance_work_mem = 1024
fsync = off
wal_buffers = 32
commit_delay = 500
checkpoint_segments = 10
checkpoint_timeout = 300
checkpoint_warning = 0
effective_cache_size = 165 000
autovaccuum = on
default_transaction_isolation = 'read_committed'

What do you think of my tunning ?

Best regards.

O.A