In one of the code-word projects I saw an $25k system with an OLTP dataset do 1 billion rows in a second.
Edit: We also had statistics that said that 90% of our customers had less than 100GB of data. 99% had < 1 TB. The vast majority of database users shouldn't even be thinking about looking at non-RDBMS systems.
If your dataset fits in memory on one node, I'm all about using a database (prefer MySQL personally, might look at postgres now that it finally has replication).