You shall need additional Disk resources for each of your new instances Log, data, tempdb.
You shall also require additional IP addresses both Private and Public, as well as ensuring there are free Network cards on the server for the additional IP addresses and a SQL Network Name for the new instances
The SQLCAT team has a good article located at http://sqlcat.com/sqlcat/b/whitepapers/archive/2009/07/08/sql-server-2008-failover-clustering.aspx
You will also want to ensure that Max Memory has been configured on each of the SQL instances and that in the event of a failover 1 Node can handle multiple resources