Microsoft SQL Server 2008 R2 Express Edition is a great database engine that is more than capable of handling most small business database requirements. There are however some limitations. The details below outline the main limitations in SQL 2008 R2 Express Edition.
1 CPU (based on a socket). No limit on cores.
Maximum of 1GB for the database engine.
(Note: SQL 2008 R2 Express Edition with Advanced Services includes reporting features, and has a limit of 4GB for the reporting services. It still has a 1GB limit for the database engine).
The maximum database size is 10GB.
(Note: this maximum database size of 10GB in SQL 2008 R2 has been increased from 4GB in SQL Server 2008).
For a full rundown of the features that each SQL 2008 R2 editions have included, you can visit http://msdn.microsoft.com/en-us/library/cc645993(v=sql.105).aspx