The MPS4 Music Port Server, and the Music Port, each provide up to 4 sources to the GC. The GC then distributes those sources to the different zones. If that is the only source component that you have, then you can have only 4 sources coming from the GC, playing through any/all of the zones.
The Music Port (not the MPS4) takes the audio output of a PC and routes it to the GC multichannel amplifier. Nuvo recommends a dedicated PC. The PC needs 1 or more serial connections to the MP. If the PC sound card has 5.1 output, then the PC can provide 3 sources. If it is 7.1, it can provide 4 sources. People seem to be having trouble with MP setup and operation if only some of the outputs are used (e.g., 3 of 4).
The setup of the Nuvo GC is not something the average person can do, one needs to spend a good deal of time figuring it out. Most people don't want to bother. Yes, you can buy the equipment yourself, but if you want a pro installer to set it up, expect to buy the equipment from him. He'll probably sell it for less than MSRP, but it will be more than wholesale. It's part of the agreement, when you use a pro. He needs to make a minimum amount of money from each job. If he doesn't sell you the equipment, he'll charge more for installation and setup.
I bought a dedicated PC to use with my Music Port. Serving music is all that it does. If you buy a dedicated PC, make sure it's configured correctly. You need to use the right OS, for a start. XP is ideal, I'm using Vista. I don't know if W7 would work. It wil be running 24/7, buy one that doesn't use much power. But be careful, as running 4 streams of music needs a good deal of 'power'. I chose not to use a fanless PC, but maybe they've improved by now. Configuring the serial ports was a headache for me, as I hadn't had to configure any before. I tried using a PCI card with 2 serial ports, but I failed; I chose poorly. I ended up using an Edgeport/2 USB-serial adapter. Mapping the 3.5 mm audio outputs of the PC to the GC was a little tricky, too, but I eventually got it figured out.
Or, just get the MPS4.