Sitefinity can uses SQL, MySQL, Oracale, Microsoft Access. Generally we do not recommend making changes to the database. We use Data Layer and Nolics for 3.x versions. From Sitefinity 4.0 we will use OpenAccess. However, for the metakeys you can use LongText, Boolean, Int, FloatingPoint, Guid, DateTime, Binary types as well.
the Telerik team