What caching tool to use for large Wordpress Multi-site Instance?

We have a multi-site instance with over 1400 sites on it (http://web.unc.edu). We are currently using Super-Cache and it works pretty well. I have read about server site caching tools (Read about Varnish today:http://ocaoimh.ie/2011/08/09/speed-up-wordpress-with-apache-and-varnish/ as well as Nginx: http://wiki.nginx.org/Wordpress).

Do y'all have a recommendation? Should we use more than one of these? I am looking for the fastest site load possible of course! Thanks for any tips you can offer.

~Jeff