What DB and Why?

Heavy relational data, full text searching: My/MSSQL
Frequent read, infrequent writes: noSQL (CouchDB)

PouchDB is going to change the face of web development forever. Offline apps cannot be done any other way and since My/MSSQL offer no form of native conflict resolution I don't see this changing in the near future.

Really what it comes down to is your data requirements. If you have a lot of relationships, need painless full text searching, or frequently write changes My/MSSQL are always going to be favoured. If you have frequent reads and infrequent writes (Twitter, FB, etc) Node/noSQL will destroy traditional systems in terms of performance by a very wide margin.

Node/IO.js uses OS threads to manage requests all of which are non-blocking. You can easily handle ten's of thousands of concurrent requests on a simple box as each request uses KB of RAM. Compare that with Apache/IIS which allocate several MB per request and you'll quickly see why a lot of dev's are using it for certain types of apps.

During the last World Cup Twitter set a new record of 618,725 tweets per minute. I'd love to see Apache + My/MSSQL handle that!

Learn both. Each has their use case and frameworks like Ember/Angular are here to stay.