I run a lightty/PHP server for my applications. Using AJAX/COMET, PHP hasn't used more then 3% of my cpu, its a 733 MHz via with 512 MB RAM. But thats also only like 5 connections at a time.
Currently there isn't a MySQL extension available, so some parts might need to be rebuilt for it (apache, lighttpd?, php?). Another person on the forum wanted to use MySQL some months ago but I don't know how far they got.