I'm not sure you understand the basic topology of an LMCE set up, which is the first thing you learn, hence I say "read read read"!. Start on the wiki.linuxmce.org at least read the FAQ, hardware requirements, and the basic setup pages. Display Drivers and the AV Wizard pages are very useful as well. Finally, ensure you get the right hardware to maximise compatibility - this system is highly complex, the last thing you want to be doing is fighting hardware incompatibility at the same time!
You have 2 NICs. One is "external" the other is "internal". You plug your external NIC into a switch on your existing home network - it will now get its IP address from your existing DHCP server. The internal NIC is your LMCE network. Plug it into a different switch, and it will provide DHCP (plus all the extra things it uses DHCP for) to that network, never the twain shall meet!
Now move whatever you want to interact with LMCE onto the second switch, et voila! Done! They will now be served by LMCE. So PCs that may have media on them, move here. NAS devices, here. Systems you intend to use as Media Directors, here. VoIP phones, here. Units for running Orbiters (PDAs, webpads, etc), here. And for simplicity, you might as well move associated equipment like networked printers, so that they can easily be accessed by your PCs - they will just see it as another network and continue unchanged. LMCE will provide IP addresses and route traffic for things like printers, but otherwise ignore them.
As you can probably now see, you might as well just move everything from your existing, "external" network to the "internal" network as in practice it will do nothing to your environment. If you do that, then you can plug the "external" NIC directly into your broadband router and not even bother with an external switch.... or even reuse that switch as your internal switch... wow! see what I mean? no drama after all...