Ask HN: how to build large scale infrastructure with less spending?

I'd say get started with one instance and go from there. You will be able to first increase the instance, second separate the database out onto a 2nd instance and third, add instances.

Two things you might want to consider or DotCloud which takes some of the pain out of scaling and Joyent which has a free year of hosting for preferenced treatment for Facebook apps: http://www.joyentcloud.com/developers/free-facebook-develope...

definitely helpful any thoughts on databases?

Have you considered using one of the PHP PaaS solutions like dotcloud, phpfog, pagodabox or cloudControl?

I am using php + mysql so thinking to get 2 small EC2 instances from Amazon.

Caching! Check out Varnish, and memcached for various caching schemes.