Oracle on a single system doesn't scale much better (if even) than PostgreSQL on a single system. Oracle's cluster solution is nice, but the expense is crazy.
Yep. I just have to add that I'm following the development of Postgres-XC (that's "extensible cluster") with more than a small bit of interest because of this, even though they're still missing quite a bit of functionality that'll be required for a 1.0 release. Even the improvements to the replication solution they delivered in 9.0 (that "Active Data Guard"-ish thing) that they've got planned for 9.1/9.2 is tickling my interests since it'll mean that PostgreSQL will have a good solution to things where previously the crazy expensive Oracle solutions were the only really nice ones.