Why dual network cards?
If you have multiple computers connected in a network, every computer needs to have a unique IP address that identifes it on the network. The DHCP server is the device on the network that has the responsibility of assigning IP addresses. Normally one computer or device--and only one--will be the DHCP server. Whenever a computer or other device is plugged into the network it will normally ask the DHCP server for a unique IP address. In a typical home network, the DSL or Cable Modem acts as the DHCP server and gives IP addresses.
If you want to use LinuxMCE's plug-and-play, which allows you to just plug any device into any jack (camera, phone, media director, etc.) and have LinuxMCE automatically configure the device, then the LinuxMCE Core needs to be the DHCP server. The same is true if you want the media directors to be dual purpose media director appliances and normal PCs.
If you would like to use your existing network as the home automation/multimedia network, you will need to make one change: since your existing cable or DSL modem is configured to be the DHCP server, you need to disable its internal DHCP server and let the Core be the DHCP server (there can only be 1 DHCP server on the network). If you have a dual network cards Core, this is not an issue. Hookup your internet connection (DSL/Cable modem, etc.) to one network card. This is your 'external' network where the internet comes in. And connect your other network card to the Ethernet switch where all your media directors and other computers are connected--your 'internal' network. The DSL/Cable modem is then the DHCP server for the 'external' network, and the Core is the DHCP server for your 'internal' network. If you don't understand this, don't worry--the Core will configure itself for you and you won't need to change any settings in your DSL/Cable Modem.
Another advantage of dual network cards is that then all internet traffic is forced to go through the Core. The Core then handles allocating your bandwidth automatically, giving higher priority to telephone and video calls so the connection is very smooth. It's all automatic if you have dual network cards at the time of installation --there is nothing to configure.