Is there a general rule of thumb to follow when storing web application data to know what database backend should be used? Is the number of hits per day, number of rows of data, or other metrics that I should consider when choosing?

My initial idea is that the order for this would look something like the following (but not necessarily, which is why I'm asking the question).

1. Flat Files
2. BDB
3. SQLite
4. MySQL
5. PostgreSQL
6. SQL Server
7. Oracle