I think this is one of the best parts of rails, It's fairly database
agnostic and you can code with a high amount of disregard for what
database is on the back end. If you write against one server and
decide to change, you can do so easily.
Most people are using MySQL with InnoDB tables, or PostgresSQL.
PostgresSQL is the way to go if you want stored procedures, as
there's a rich amount of language support in Postgres, otherwise,
just use MySQL.
-john