Which RDBMS best fits my need?
My recommendation is to start out with Postgres.  The advantage is that it has a full feature set and it teaches you good habits from the start.  Once you are familiar with it, it is easier to move to up to the Oracle/DB2 approach or down to the MySQL approach easily enough.  On the other hand it is an advanced database and you can take it extremely far if you want.  And it is open source to boot.