Fitting the bill perfectly..... (Score 1)

I am/was in the exact same boat, and I will attempt to share the info and decisions that I made.

First off, read the below past comments on this article. It ran just recently, and will answer alot of your questions concerning maximum suggested length of audio cable to run, among other options for wiring.
Best Options for a Home Entertainment Network?

Second, you really have to analyze your scenario (i.e. length of audio cord, zoning, etc.) to determine how you want to serve and interface with your MP3 server(s). My opinion, run as much damn ethernet as possible....lots and lots I tell you (unless of course you just install smurf tube). Additionally, dont be silly and attempt to control the volume from the server software. Keep your recievers at a nice level, and install inwall slider switches for volume control to the speakers.

Now to go off the deep end, I will describe my scenario, and all the n-th opinions that I debated with.

I wanted 3 zones in my home: Basement, Kitchen/Living Room/Dining Room, Master Bedroom/Bathroom. Each room has its own set of square inwall speakers, or round ceiling speakers; however are clumped in the zones as described. Server must be out of site in the basement wash room (Wife requirement 1.001.A).

So after measuring how much audio cable I would have to run from the basement to the 3rd floor master bedroom/bathroom, it was time for a new plan (the other zones are no problem since the other zone is directly above the basement wash room).

My scenario is that I originally wanted one MP3 server with multiple audio cards. After many nights giving this a try with XMMS, WinAmp, and Windows Media Player (8, 9, 8-SDK, 9-SDK) I came to the conclusion that it is a pain in the butt. This is especially true if you are using multiple brands of audio cards and/or older machines. Additionally, this still does not solve your multiple reciever problem (or buying a little more expensive mulitple input/output reciever). If you have multiple older machines, use those for multiple severs, *not* client selection machines....I will get to this later.

Because I had such a stretch from the server room to the bedroom zone, I decided that I will need to have another server in the bedroom. I toggled between the option of either a Hush Computer or a Audiotron. Both are silent, small, and look nice in your TV cabinet.

Even though the AudioTron, Winamp, XMMS, all have APIs; I decided a long time ago that I would write my own MP3 server software with WMP SDK and DirectX (so guess my choice of OS). Because I have complete control of the software I created my audio network as such:

• Lots and lots of ethernet
• Small audio network in the Master bedroom/bathroom. 2 rounds in the bedroom, 2 rounds in the bathroom, Hush computer in the TV stand which recieves remote software commands (like timed events) from the server and utilizes a network share of all the music from the server. My next task is to write the front end, hook up the SVid out to the TV, and hook up some IR controls such that I can control the MP3 selection with my remote (Sony Commander). Having the second (loose term) proxy/dumb server, solves my multiple server/audio cards problem. IMHO, if the audio cable is to far to stretch, link a middle computer with the server with your ethernet, and consider making your zones as large as the maximum length of strung audio cable (hence you never have a server with multiple audio cards). Yes, I know that it may suck that you may not listen to different music than a room that is 5 feet away, but deal with it, it is a small price to pay (this was my sacrifice of clumping together the Dining Room/Kitchen/Living Room; however this works out really slick)
• Basement Washroom/Server Room. A tad beefy, but a 1.4 ghz machine as my server. Since the server is down in the basement it is no big deal to hook it up into whatever I decide to put in the basement.
• Kitchen/Dining Room/Living Room. This is where I am currently debating some options. I have 2 square inwall speakers in the Living Room, and 2 round cieling speakers in both the Kitchen and Dining Room, and 3 slider switches for unique volume control for each set of speakers. As said before I am writing my own APIs and UIs so I want the same selection ability that I have in the bedroom (noone wants to run up and down stairs to flip music). Whatever, this device is, it will be on the counter top of the kitchen, doubling as a recipe holder application. Now, I do not want a full blown computer in the kitchen, but I do want something touch screen. Some options would be SlimMP3, Microsoft SmartDisplay, AquaPad, PocketPC, or a very conceled Mini-ITX computer with a touch monitor.

[Shameless Outcry] If anyone has an opinion of what I should use in the kitchen to interface with my playlist, please reply.

Currently, I am leaning towards the SmartDisplay, but some reviews are sucky, and it is expensive. CE devices are to small, slow, and memory limited (I dont want to be shuffling album cover art across the network onto a CE device). Mini-itx.com has some neat hacks, I may lean towards that option if i grow some hair on my nuuuuut..... *cough* chin.

I hope this helps you and many others starting these types of projects.