Do I need to buy another licence for Microsoft SQL Server 2008 for failover?
No. A passive server does not need a licence, provided that the number of processors in the passive server is equal or less than those of the active server. The passive server can take the duties of the active server for a max of 30 days. After that, it must be licensed accordingly. Database mirroring and failover clustering is available for Standard and Enterprise editions of SQL Server 2008.