For anyone who is interested, Tim Perdue has performed a set of
benchmarks comparing MySQL with PostgreSQL
See his article at
http://www.phpbuilder.com/columns/tim20000705.php3?page=2
and
http://www.phpbuilder.com/columns/tim20000705-res.php3
for the raw results.
For read performance, they are comparable, with mySQL performing about
50% better.
In the 10% write case, however, Postgres kept going beyond the
client-concurrency load where MySQL falls over.
-- Neil