insert speed - Mac OSX vs Redhat

I've read most of the threads on insert speed in this list and wanted
to share some interesting observations and a question.

We've been benchmarking some dbs to implement Bayesian processing on an
email server. This involves frequent insert and updates to the
following table:

create table bayes_token (
username varchar(200) not null default '',
token varchar(200) not null default '',
spam_count integer not null default 0,
ham_count integer not null default 0,
atime integer not null default 0,
primary key (username, token));

On a variety of hardware with Redhat, and versions of postgres, we're
not getting much better than 50 inserts per second. This is prior to
moving WAL to another disk, and fsync is on.

However, with postgres 7.4 on Mac OSX 10.2.3, we're getting an amazing
500 inserts per second.

We can only put this down to the OS.

Can anyone shed light on why Redhat appears to be so much poorer than
Mac OS X in supporting postgres insert transactions? Or why MacOS
appears to be so much better?

BTW, on the same hardware that postgres is running on to get 50 inserts
per sec, MySQL (4.0.17) is getting an almost unbelievable 5,500 inserts
per second.

-SL