Ludek,
I've run OpenACS on both Oracle and Postgres. The Oracle install is a 2 CPU box, performs adequately. The PG installs are smaller configurations. I definitely consider PG a "mature and reliable" alternative to Oracle. The thing you get with PG over Oracle is easier administration and faster recovery in case of crash. Oracle can be a bear to administer, ESPECIALLY in a clustered (RAC) configuration. Note that PG clustering is new (as of 8? Someone please correct me), and Oracle took 3 revisions to get it "right" (some people will argue successfully they dont even have it right in 10g).
