The virtual network is great for connecting two or more virtual guests directly. Its fast and doesn't add traffic to your physical lan.

If you have a virtual guest that gets a lot of traffic, then giving it its own network card (nic) can help speed things up as well as keep the heavy traffic away from the virtual guests and your users - those that don't need to be fighting against a steady stream of packets.

The more nics the better. I tend to use three because I occasionally move large amounts of files, and I don't want to slow down other things on the lan. A decent gigabit switch means never having to share the bandwidth.