I posted a pretty detailed answer on stackoverflow about when it is appropriate to use relational vs document (or NoSQL) database, here:  

**[Motivations for using relational database / ORM or document database / ODM][1]**

Summary:

- for small stuff, go with whatever tools you are familiar with

- a few gigabytes is definitely small stuff: it doesn't get big until it is too big to fit in a single [MySQL Cluster][2] with a reasonable number of nodes (16-32), which means maybe 8-16TB data and a few million transactions per second (or a more conventional hard-drive-based database with up to 100's of TB data and a few thousand transactions per second).

- if you're stuck with another database (not MySQL Cluster), get more mileage out of it by throwing in FusionIO hardware.

- once you have data larger than a few TB and faster than thousands of transactions per second, it is a good time to look at moving to logical sharding in the application code first and then to NoSQL.

- [Cassandra][3] :)


  [1]: http://stackoverflow.com/questions/13528216/motivations-for-using-relational-database-orm-or-document-database-odm/13599767#13599767
  [2]: http://www.mysql.com/products/cluster/scalability.html
  [3]: http://cassandra.apache.org/