There are reasons to encrypt in the database, protects data in backups, loss of database, etc. However if it's a big deal, the wire should be encrypted as well. There's less concern of that with internal applications as most networks are switched, it would require a few people to trace the data on the wire, etc.

There's no way to tell what the load will be. The CPU will have to decrypt and encrypt, you have limits on some encryption with data sizes (asymmetric keys), you don't necessarily want to encrypt all data, etc. It requires some testing to determine the load for your application.