If you have a lower traffic site I don't think you will see any difference between the two.  Have a look at:

http://www.microsoft.com/sql/prodinfo/features/compare-features.mspx

which shows the differences between the editions.  One of the big differences between express and the other editions is the cost of licensing for use on the Internet.

As to remoting to the database you can do this using the Express edition of the management studio or if you have a valid license for the SQL 2005 Management Studio you can use this to connect to Express.  if you are doing this over the Internet I would suggest you read up on the security configuration of SQL to ensure no one else can also access your database.

HTH

Antony

In thinking about it, I am not using any advanced features of MSSQL 2005, so the express edition would clearly suffice; if, however, the forums and data in the database grew so large as to justify a SQL 2005 database, would migrated to a new database in an existing portal be easy?  I am just a dumb economist and don't have a Master's in information management systems, so want to make sure a dummy could upgrade down the road if scalability is an issue.  :)