You *should* have two NICs. One for the physical network and another for your Management interface.
The reason is that as you move forward and you may endup have two or more ESX servers in your farm. When using products such a Virtual Center, the managent interface would be used by virtual center to manage your multiple ESX Hosts and Guest OS's as well as move them between hosts for failover.
Lastly, it is recco'd for security that your management interface be segmented from the rest of your network.
