Yup - my first inclination would be MySQL or Postgres with ROE to
access stuff. Maybe both an OODB and an RDBMS - our 444.NET search
engine for example has all the raw bulk search engine data in MySQL
(using MySQL's full-text indexing for the searching), and all the
complex domain data like customers, subscriptions, etcetera in
OmniBase.
The stuff we put into MySQL is so simple that we haven't looked at
complex O/R mapping stuff, just handcrafted queries.