Install additional instance in a cluster

I am looking for a best practice or any suugestion on the best way to install an additional Sql 2005 instance in a cluster environment. We have a production environment that is active/active/passive and need to install an additional instance with a little impact to the current production instances as possible. Ant suggestions or articles I can review?

Hi...see the whitepaper at the following address:

http://www.microsoft.com/downloads/details.aspx?FamilyID=818234DC-A17B-4F09-B282-C6830FEAD499&displaylang=en

It contains multiple sections and information on installing Sql2k5 in a clustered environment...

HTH