> 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.
True but scaling vertical is really a luxury, in most situations especially free to play or open systems for a certain level of usage, this is typically not economical. For a good vertical scaling strategy you need servers that reach into the tens or hundreds of thousands of $$.
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.