James Fuller has posted a guide to scaling your web application using the nginx web server, memcached, PHP-FPM and APC caching.
We sell an educational product that serves a predictable 15,000 requests per minute for 10+ hours/day, every day. Instead of Apache, we use nginx with PHP-FPM to handle this traffic. This is becoming a very popular setup for many companies with non-trivial traffic, but I have also found success with it in my small 256MB Ram VPS. For various reasons, nginx does a better job with memory and concurrent connection handling than Apache. In this post, I want to talk about some of the reasons you might want to go with this setup.
He talks about some of the efficiency gains that memcache and nginx can give you pretty easily and some of the common uses for nginx, including using it as a reverse proxy. He talks some about Apache's typical request handling and shows the difference between that and how nginx does its "never block, finish fast" handling. He fits in the other pieces - PHP-FPM, memcached and APC - showing how each of them offers their own types of performance gains for different areas of the application.