Sizing your Rails application with Unicorn on Heroku
This tutorial assumes you are running a Rails application on Heroku with a Unicorn server. However, if you are not using one of those you will find some general information which can help you in your context.
This article will help you to correctly configure your Unicorn server based on real data.
I recommend you to use New Relic, which is really easy to setup with Rails & Heroku. It will help you monitoring your application and configure it at its best.