If you have SQL Server 2005 Enterprise edition you can simply add the new nodes, install the required SQL Software (and any other software that is needed) move the SQL Server to one of the new nodes and power off the old nodes. After they are power off simply evict them from the cluster and you are done.
If you have SQL Server 2005 Standard edition you can only have two nodes in the cluster at a time. Because of this, you’ll need to remove one node and evict if from the cluster. Then add one of the new machines to the cluster and install the SQL Server software on it. Then fail over to the new node and remove the other old node. Then add in the other new node and install the SQL Software on it.
If done correctly the only down time that you will see will be while the SQL Server is failing over from one node to another which typically has a down time of 3-7 seconds.