PostgreSQL will take you pretty far. On the very high-end, you have products like Oracle and DB2 and (while painful to discuss) MS-SQL server. Feature-wise, Postgres stacks up pretty well (though lacks things like built-in replication, built-in clustering, etc.), but as ChrisA mentions you can't necessarily get the same sort of support. You can certainly buy postgres support contracts from 3rd parties, but most wouldn't consider it "the same" as buying this sort of support from the vendor.
I have a bias that I should reveal, my company has very large deployments of postgres in a clinical setting. I do understand the shortcomings, however, and we have certainly investigated a move to oracle, etc. The expense is obviously a very large deterrent, though we have spent many man hours on jury-rigging real-time-ish replication, etc.
-Lee