All very good questions indeed, it all depends on you, and what you want to accomplish next. i was at the very same crossroads myself a few months ago..
Due to the nature of my business i opted for PHP/MySQL to strengthen my back-end programming. I hear good things about RoR too, actionscipt is a bit of a question mark as it not really sustainable.
And Yes, Node.js can interact wit mySQl.
Check out node.js module list
node-mysql — A node.js module implementing the MySQL protocol
node-mysql-native — Yet another pure JS async driver. Pipelining, prepared statements.
node-mysql-libmysqlclient — MySQL asynchronous bindings based on libmysqlclient
node-mysql-pool — connection pool for node.js on top of Felix Geisendörfer’s MySQL client node-mysql.
All in all, keep learning what suits you the most.. I chose php/mysql and i'd recommend the same.