Unfortunately my main site hasn't given me an excuse to tinker with the setup, so I'm going to ask here. Right now I use Apache 2.2.x in front of a pack of Mon

Unfortunately my main site hasn't given me an excuse to tinker with the setup, so I'm going to ask here. Right now I use Apache 2.2.x in front of a pack of Mongrels and mod_cache is enabled to cache content based on Expires headers. (I tried Passenger over the summer but it still had issues at the time). Now with mod_cache, mod_proxy_balancer and my mongrels I have a very stable setup with only two programs. What sort of benefit would there be to using nginx and say varnishd along with mongrels? I know varnish is a much more flexible cache than mod_cache, but would there really be a performance boost of any note?