You seem to be confused about the term n-tier - the n is not a function of the number of servers in your hardware architecture but of the design of the application. You already have at the very least a 2-tier architecture in your application in as much as you have a program (probably combining presentation and business logic layers) and a data layer in your RDBMS.

Splitting the separate application tiers onto different servers is not necessarily going to enhance the performance of your application in itself, but will give you a number of opportunities whereby the performance can be increased, say by optimisation for the particular role of the server.

/j\