For audio only, essentially the only program you'll be running is Squeezebox Server, so I suspect the basic hardware configuration (entry level CPU, 2 or 3 MB RAM, HD of your choice) will be more than up to the task. Salvage an abandoned monitor and keyboard and you be off and running in no time.
My friend opted for the wireless card, but you're probably better off it you can wire it to your router.
My friend opted for the wireless card, but you're probably better off it you can wire it to your router.