Do I lose anything by using legacy databases?

If I have information in a legacy database will I be losing anything
by using RoR. Can I store configuration information in mysql but
display data from my legacy database?

That really depends on what database it is. However, there's
absolutely not requirement for you to use MySQL; ActiveRecord provides
adaptors that will work with MySQL, PostgreSQL, SQLite, Oracle, DB2 or
SQL Server, and there are also adaptors available for other
databases.

While there are a few DB-specific issues to bear in mind, you should
be fine using any of these databases.

-Matt