Hopeful,
Yes, there is much, much more to it than that, also including system noise, disc i/o, dac buss i/o and so on. Even in the windows world, there is and can be large differences between all the i/o protocols you mention, even between asio drivers. And the mac mini, even without any special tweaking is worlds ahad of a fully optimized windows machine. I know, I've donethis for quite a while now and had my little asus EEE box running xp home that had been stripped down using nLite, with both kernel streaming and the AQVox asio driver (talks directly to the usb hardware and bypasses the windows mixer altogether, and still, the mac mini running play.app -- a freebie from the same guy who wrote Decibel that Grainger just mentioned, easily clobbered the fully optimized windows machine.
Computer audio looks simple enough on the outside, but is vastly complex and can take quite a bit of "tuning" and optimizing to get the most out of. My mac mini with an SSD, 8 gb ram, the music files on a firewire external drive, and the mach2 music operating system overhaul, and this is easily the best transport I've had and can go toe to toe with, and even surpass some of the best transports/players out there.
That said, in second place, I'd go to a small, single board linux computer -- the PCEngines ALIX board, which has no video, keyboard, disc interfaces, etc., just a compact flash cad for the operating system and MPD software. You then pull your files off a NAS or another computer (mac, pc, or linux) in your home and on the network, and then use a client to control it. This is about the best price/performance digital transport out there as the alix board with case and power supply can be had for about $120, and the software is free. There are clients for all kinds of hosts -- netbooks or laptops, smart phhones, iPod Touch, iPad, and even one that uses nothing but a mouse.
I just yesterday got the last parts I need to build a super regulated linear psu for the alix and that should take it up another few notches, and who knows, may even give the fully tweaked mac a run for it's mney.
Yes, you can do as you say, just take a functioning windows machine (or mac for that matter) choose a player (I too used foobar for several years) and connect it to the dac and get music out, but when you've taken the time to find andoptimize everything between the computer, disc storage (file storage), and dac, the results can be nothing short of incredible.
The one thing I can upgrade about my mini is to put it on battery power and use a special firewire cable with no power leg, but that's a whole lot of money I don't have right now.
Sorry if this is not what you wanted to hear, but I've done this, one change at a time,and in all kinds of combinations and permutations and I promise that whatever you think of computer playback quality can be, well, it can be much, much more than that.
-- Jim