One of the ideas behind using Nginx (or Apache for that matter) instead of the built in "Jetty Web Server" is that you can alleviate some of the strain to bukkit that may occur due to high traffic etc. to the map. If you host your own website (and therefore are already using port 80), it will also allow you to have the map sitting on a nice http://map.example.com/ as opposed to http://map.example.com:8123/ (setting up DNS records is past the scope of this document).
Note: This guide is made with the assumption that the web server is seperate to the Minecraft server. In this example I will use 192.168.1.2 for the Minecraft server and 192.168.1.3 as the Nginx server. If you choose to run them on the same server you could just change the 192.168.1.2 to 127.0.0.1 in the configurations - just be aware that this is likely not an optimal configuaration (questionable if proxying in the situation would help?).