552505 Setting up MongoDB on EC2 to Auto-Scale

I currently run my website on a single server with a single MongoDB. I am moving to Amazon EC2 and building a horizontal scale-out model where more web-servers are dynamically started up. This poses a challenge for how I use MongoDB. I have a crawler/indexer that read/writes to the DB and a web-server that read/writes to the DB. I want to change my architecture so that I have 3 server types (crawler, DB, web machines). This task is to build the MongoDB on Amazon EC2 so that it scales horizontally as I add more data and/or the CPU load is too high.