I would recommend building a PC to your liking in terms of size and cosmetics that would blend in with the rest of your system. Personally, I just can't see building one on your own when you can get a beautiful Hush PC with ultra-quiet operation and stunningly good looks for less than $1,000. On the Mac OS side, there is the Mac Mini for less than $700. However, if you have the itch to roll your own, that's cool too. Add to that a Network Attached Storage (NAS) RAID system which you can either build or buy ready-made, and you're in business. Set-up the PC first as a network node and than connect the NAS RAID via a 10/100/1000Base-T Ethernet network connection. I would suggest placing the NAS RAID in a different room far from your listening room/area, so you won't be distracted or annoyed with hard drive or fan noise.
In building your own PC-based system, stay with an Intel chipset-based motherboard, if at all possible. Why? Most hardware and software manufacturers develop their products using Intel chipset-based systems. Intel's 955X Express and 945P Express chipsets are good, stable chipset platforms to stick with. For the most part, AMD processors and alternative chipsets are also fine for most PC audio operations, but there may always be that one application or piece of hardware that could pose a compatibility issue. Personally, I prefer using a chipset that supports the use of a dedicated graphics processor like the Intel 945P. This just frees up the CPU and system memory to do the business of managing the digital audio in yor server so you won't get skips, stutters or other extraneous digital artifacts. If your music server is used just for music playback and nothing else (no media player visualizations, no digital audio editing for vinyl transcriptions, etc.), then a dedicated graphics processor and memory is not required. I would check with Newegg.com to get an Intel chipset-based motherboard and appropriate Pentium 4 processor. For RAM, get the fastest DDR RAM up to between 512MB and 1GB from a manufacturer like Kingston, Crucial or another recognizable company.
You'll need some way of sendng audio from your motherboard, either via digital or analog. Most audiophiles hate the idea of using a DAC that is integral to a PC soundcard or motherboard for sending out analog audio, and prefer using the soundcard or motherboard to serve only as a digital audio output interface. The claim is that the electromagnetic noise within a PC enclosure environment is detrimental to the quality of the analog audio that can come from a soundcard DAC or motherboard DAC. The general consensus is that an outboard DAC performs better than an integrated DAC on a soundcard or motherboard DAC inside the PC. There are good soundcards for S/PDIF digital audio output from RME, Digilabs, M-Audio, Echo Audio, E-MU, ESI, etc. I would shy away from the mass market stuff from Creative as their soundcards process the digital audio rather than passing it through. There are also motherboards that have built-in S/PDIF digital output jacks. If you don't want to use S/PDIF, there are a few audiophile-grade USB DAC options that work well with the USB 2.0 bus: Wavelength Audio Brick, Apogee Mini-DAC with USB option, Perreaux SXD2 USB DAC and the S2S Audio TwinDAC+. More manufacturers will be coming out with USB-compatible DACs and USB audiophile-level digital audio interfaces. Again, this is where a good motherboard with stable, error-free USB bus controllers is a must. Whatever you decide, just keep in mind that you want to minimize jitter.
For myself, I am waiting for the new Intel-powered Macs to come out. I think Mac OS X is better suited for handling digital media than the PC (I am currently a Windows XP Pro user), Macs are quieter PCs out of the box and the performance-to-cost ratio favors Macs. I am less interested in dealing with hardware driver and compatibility issues than with enjoying my favorite tunes on a good hifi system.
