Database suggestion for a social network/ knowledge base community? I am looking into various database types and DBMS's for a new project I am wanting to start in the summer.

I have built systems in MySQL and postgreSQL , now I am wanting to expand my knowledge and experience in Databases.

My project will be a type of social network / aggregate knowledge thing. (still havent developed a term to describe it yet).

I have been looking at:


Cassandra (use its own type of query language); It seems to be good for feature rich content and delivering high performance query execution. However I am not too keen on it because it requires a java environment to work on and I would prefer to have nothing to do with Oracle.
MongoDB (noSQL type of DBMS) ; great scalability however you lose all the capabilities already available on the proven SQL language like business information queries.


Requirements of the system:


Data Text, dates , times, xml, small ints, blob, 
Structure/behavioir: normalised 3NF, non realtime, relational , scalable, robust
Environment: unix/linux , no JAVA!, preferably run on C


I was wondering if you could point me to any other Database systems that I should research into.

I have also had a look at Object Relational Databases , I quite like the idea of them working with PHP objects (PDO's) however their performance seems a bit poor.

Seeing as there will be DBA's here, any feedback on these systems that you have operated would be appreciated.

Thanks
Consider also that there is no reason why you can't use a relational database for some things and the nosql database for other things.