I would run a PC platform. Mac no longer maintains a software advantage and certainly doesnt have a hardware advantage. As a matter of fact, Macs use converted PC technology in every hardware aspect except for CPU and motherboard. Upgrades, parts, and initial purchase will all be cheaper with the PC platform.
M-Audio makes very popular soundcards at attractive prices. My recommendation would be use an outboard DAC and feed it via the Digital out (spdif) from the computer. So the ultimate soundcard wont be an absolute. Of course if you dont plan on a DAC, then the better cards will provide better analog outs. I use a lowly Audigy 2/Theta DAC at the moment and cant really complain.
Memory is more of persoanl preference. Basic sound reproduction doesnt require a lot of system resources.
If this server is to be located in your listening area I would recommend a quiet approach. Maybe a low core voltage CPU that wil run cool and wont require CPU fans or case fans. Whirring fans you probably dont want. Check out the powersupply as well, they usually have built in fans.
As of right now I use an old program called "Siren" it was from a sound engineering software company called "Sonic foundry." They may still make it, it is OK. There are probably better music serving programs available now. It can call up the names from the internet so you dont have to type them all in. I think that is a paid service, though I am not sure.
I hope this helps...