apache resource usage

A friend of mine asked me recently what setup I would suggest for seperating static content serving and dynamic content serving. The main concern being that dynamic content servicing under apache 2 is more resource intensive than static servicing (pretty obvious eh?). As far as static content delivery, there are far simpler web servers availible that can spit out data just as fast as apache for a lower resource cost (boa, lighttpd). Now you've got two web servers, one httpd for dynamic and one for static, but how are you going to make it seem like one site? mod_rewrite (lighttpd aparently supports url rewriting, so it might also do) to the rescue. If you toss apache in front of those two servers (possibly running on the same machine but sitting on different ports) then you can have that main apache process act as a traffic cop routing requests to the httpd that can service them.

That was just my first shot at the issue, if anyone else has ideas please send them on over.