For small things I like DBD::SQLite, and I expect that one of the advantages will turn out to be easy 'upgrading' to a stronger DBMS (I have just started using SQLite). Apart from heavyweights Oracle and Sybase, there is also a MySQL choice of different underlying engines: InnoDB for heavier work, MyISAM (and other types) for lighter and faster use.

DBMS will always have the advantage of scalability, and if a database (or enough index) fits into memory, performance will be all right anyway.

Maybe I suffer from the same affliction, but I would say RDBMS is a very versatile hammer to cast problems into nails with.