> It happens with large RDBMS systems anyways after about 5GB of data, same type of scalability decisions have to be made.

I work every day with a > 4TB database and its all on 1 machine (a very big machine) with some very fast SAN. If you have money, Oracle can get very big on the right hardware. I am willing to bet MySQL or Postgres could do very well on that sort of hardware too.