I would recommend PostGreSQL and you probably won't need to switch to Oracle if you use PostGres properly. Importantly its free and has a usable and friendlier license than MySQL.
I am working on a startup idea which will also have high traffic and I am personally using Schevo - which is a non-sql Object DBMS for python based on Durus. I like Schevo because SQL sucks and using Schevo is like you a programming language - in this case its Python. You can check out the Schevo project at www.schevo.org. This would work for you only if you are using Python (highly recommend). Schevo does not have very good documentation yet, but it has excellent support on the IRC and Google Groups from its creators - at least I have found them to give complete and quick responses.Hope this helps.
I am working on a startup idea which will also have high traffic and I am personally using Schevo - which is a non-sql Object DBMS for python based on Durus. I like Schevo because SQL sucks and using Schevo is like you a programming language - in this case its Python. You can check out the Schevo project at www.schevo.org. This would work for you only if you are using Python (highly recommend). Schevo does not have very good documentation yet, but it has excellent support on the IRC and Google Groups from its creators - at least I have found them to give complete and quick responses.Hope this helps.