Thanks till! By your description then it would seem that the web server and database server are on the same machine? How much ram do these machines have?
I see your using Apache for the static content as well. I was on Lightspeed's site looking at their server offerings recently and they had a chart showing the requests per second being served up by it in comparison to others. It showed Apache2 up along the smaller servers such as Lighttpd.
I'm coding a social networking site from scratch so I'm wondering what sort of online numbers I'll be able to support. I'll be utilizing memcache and other optimizations such as xcache to push the hardware as far as possible. Your insight on the hardware used here has been very much appreciated.