Apache Cassandra, do I need more disk space?

What is the relationship between storage space, Random Access Memory (RAM) and CPU with respect to apache Cassandra? According the apache Cassandra 2.1.2 node configuration, maximum recommendation capacity is 3 to 5 terabyte per node, if am running this maximum load of data, will I have to increase the RAM? If the RAM is not increased, will it affect the throughput and latency?
