I agree, varnish in front of nginx might be a bit overkill. We use varnish because we run apache, and I'm so infatuated with it that I recommend it to everyone. I've seen it pull off thousands of requests a second without breaking a sweat, then again nginx is no sloth either.
We are considering switching from apache, but the current setup works so well that we don't want to mess with it :)
We are considering switching from apache, but the current setup works so well that we don't want to mess with it :)