I've created PHP application for Facebook. It uses MySQL, Memcached and works on Lighttpd on Centos 2,6 Ghz and 2 GB RAM.
it's basically one PHP file that after first run is cached and every next time it's served in 0,8 ms directly from Memcached.
What more can I do to shorten this time?
Once you get to 0.8 ms, I'm not sure you can go any lower.
However, you can set up multiple servers to handle many concurrent requests (with a common memcached). You will then be able to scale very high by simply adding a new server when you reach limits.