I posted a pretty detailed answer on stackoverflow about when it is appropriate to use relational vs document (or NoSQL) database: http://stackoverflow.com/questions/13528216/motivations-for-using-relational-database-orm-or-document-database-odm/13599767#13599767
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 with a reasonable number of nodes (16-32), which means maybe 8-16TB data and a few million 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, it is a good time to look at moving to logical sharding in the application code first and then to NoSQL.
Cassandra :)