the best way to guess is to use the specifications in the Vicidial Manager's Manual. Free downloadable manual on EFLO.net.
Anybody who says anything requiring less hardware than what the Manager's Manual suggests ... has probably never dealt with an enterprise-level system (or perhaps only ONE). Individual servers are difficult to predict even with full knowledge of the scenario, but a cluster with 150 agents and 1500 calls? Not predictable.
Make sure you have at least two servers for every role. With 150 agents, you should be looking at 4 dialers minimum.
Note that "DB" and "Replication/Reporting" are both database servers, so while technically there can be only one DB server, since the DB is also loaded into the replication server, which is used to generate reports without interfering with operations, I consider it similar enough to call it the second DB server.