It all depends on your current setup.
Do you use Mongrel+Apache, LiteHTTPD or Webrick? If you use Apache as
a proxy to your application server then you will need to copy any
Apache virtual domain configuration files (or mongrel cluster files)
and tinker with them to work on your new host.
Are you using SQLite or a database server like MySQL/postgres? If you
don't use SQLite you will need to dump the database to a file an load
it into the new database on the new host.
Clear anything in the rails_app/tmp directory.