I think I know you well enough now.... reading between the lines of your post, a Mac it is....
Get 4GB memory on the mac. If you want to upgrade the standard 2GB later as OS grows you will have to throw out the two 1GB sticks.
You can always use Shane's handy
USB>SPDIF adapter if you want SPDIF output from the mac. iTunes is not audiophile approved. Decide what software you will use before you decide on hardware.
Here is a Mac only music server app that a friend of mine uses with his highly modded mini. He is very fussy audiophile, uses Feastrex in OB, builds his own SET tube amps and he says it sounds great into Lynx Aurora DAC. The app allows 2 way digital crossover. This developer has appeared at many trade shows with his vinyl software, super nice guy, smart, and has a lot of good reviews on the sound and ease of use of the vinyl app. He is a real audiophile. I think Fremer reviewed it last year.
http://www.channld.com/pure-music1.htmlTaking the Devil's Advocate side of things for a moment, and I mean that literally what with Bill Gates being the anti-christ and all...
Microsoft Security Essentials is excellent and idiot proof protection from all threats. I thought Avast was fast, but SE brought a big speed improvement. I recently installed it on a bunch of Mac Minis running Windows 7, no performance hit. It is free and works very well. My kids play internet computer games and it has kept them clean when no others would. You never think about it ever again after it's installed. Even during updates there is no noticeable impact on performance. Windows 7 is turning out to be the real deal, and MSFT stock is finally emerging from the coffin after a decade because of Win7's brisk sales and broad acceptance. I would not hesitate recommending it to you, since you are already familiar with the Windows Way. You will have Mac initiation period to get used to the new interface. But if just for music server then it won't matter. Most people say it is nicer once you get up to speed. With Win7 and a decent chipset and BIOS you will not need to reboot the PC either. It can sleep automatically when not in use, and wake when you want music.
The mac will probably cost about 2x what similar feature mini HTPC will cost. They have similar Intel processors, so no significant performance advantage, nor does it matter for simple streaming. If you want to do some room correction on your digital stream or something fancy like that in the future then a PC is where the software is. Fan noise is important, check the reviews for that.
Shane and Carl both use a monitor at the sweet spot. I don't like that approach, I prefer the handheld controller like sonos, duet, iPhone, whatever. Nowadays I mostly use my laptop to control my Sonos system. If you can use your work laptop to remote control the music server might make the server installation a little more streamlined, eliminating the couch monitor. I think an itouch could control iTunes on the Mac Mini via wifi, then backend the iTunes with Pure Music app for sound quality and you could be sitting pretty!
You also need to consider where you will store your music files. If they are going on the server, then Mac mini will require you use an external USB drive, or a file server elsewhere on the network. With a PC you could make an internal RAID array in the server.
For me, a fileserver outside the music room is the best solution for storage. It doesn't need to run any music software, only serve the files, so a cheap NAS appliance like DLink DNS-323 is enough. RAID 1 protects the data. Slide in two hard drives, format, plug it into your router and you're done. A USB drive in the Mac is even easier to setup, but no redundancy.
That's all I can think of now. Except the BIG QUESTION: Will you still be our friend after you get
all cool with a Mac?