Tweakers.net has done a database performance test between a Sun T2000 (8
core T1) and a Sun X4200 (2 dual core Opteron 280). The database
benchmark is developed inhouse and represents the average query pattern
from their website. It is MySQL centric because Tweakers.net runs on
MySQL, but Arjen van der Meijden has ported it to PostgreSQL and has
done basic optimizations like adding indexes.
Arjen wrote about some of the preliminary results previously in
http://archives.postgresql.org/pgsql-performance/2006-06/msg00358.php
but the article has now been published http://tweakers.net/reviews/633/7
This is all the more impressive if you scroll down and look at the
behaviour of MySQL (after tweaking by both MySQL AB and Sun).
Jochem