Adding more nodes to a cluster is one of the biggest performance boosts that can be realized. More nodes provide the ability for the cluster to better balance client requests and to better respond to the loss of a node. If adding additional storage to existing nodes, then it may be necessary to add more RAM at the same time. For many applications, 1GB of RAM per 1TB, or per two disks of storage is a good guideline. So, for the most performance, add nodes, storage to existing nodes, and RAM if needed.