There’s a saying in the business world (OK, in obscure corners of the IT world) that goes something like:
‘A good DBA is second only to a good accountant’
Good database admins cost a lot of money, and that’s true for MSSQL, Postgres, and Oracle. Knowing how to optimize a DB is classic theory (you can take courses on it at any 4 year college, and I have), but applying that theory to the real world is challenging - you have to be able to watch a live DB, modify the structure, push the changes back to the developers, tune their queries, tune the system (occasionally rewrite parts of the system - a lot of the work on FreeBSD lately has been pushed back by Yahoo as they find limitations of the threading implementation).
Oracle is fast. It’s very fast. It also supports some advanced concepts (transactions, rollback, etc), which until recently, some of the free databases lacked (specifically MySQL). It also runs on Unix platforms, which MS SQL intentionally lacks. If you were building a very large system that had to be up 99.999% of the time, you don’t plan a base on Microsoft, unless you want to pay out the nose for some clustering server package. You can go with Oracle on Unix (Solaris, Tru64, HP-UX, AIX, Linux, FreeBSD), for a fraction of the cost and have a really good chance of hitting the uptime numbers you need.