Hi,
With the Peachtree Decco, you don't have to worry about any on-board audio on the Motherboard because the Decco has a DAC with a USB input. I'm not sure how good the USB input is on the Decco, but it does simplify connectivity issues quite a bit. Generally speaking, unless the USB DAC has a special design like the Wavelength products, you will have better results using a different digital input. Still, the better results usually require a separate sound card as the on-board digital outputs are not usually very good. So, for on-board connectivity, the USB is probably a good alternative.
When keeping a computer in the listening room, I think the most important aspect is silence. So I would follow Tmesselt's recommendation, or do some research at silentpcreview.com before building a PC.
I am kind of intrigued by the MAC mini because of its silence, but I have never tried it. Regarding PC components, Seasonic power supplies are excellent from a silence perspective, as are Antek cases. You will definitely want a motherboard that supports adjustable speed fan headers if you are not going to go fanless. It is worth it to find a case that supports 120mm fans because you can spin the larger fans at slower speeds and they will be much more quiet than a smaller fan spinning at a higher speed. Same thing goes for the CPU cooler. A large CPU cooler that can fit a large fan will be much more quiet than a stock CPU cooler.
ALan
With the Peachtree Decco, you don't have to worry about any on-board audio on the Motherboard because the Decco has a DAC with a USB input. I'm not sure how good the USB input is on the Decco, but it does simplify connectivity issues quite a bit. Generally speaking, unless the USB DAC has a special design like the Wavelength products, you will have better results using a different digital input. Still, the better results usually require a separate sound card as the on-board digital outputs are not usually very good. So, for on-board connectivity, the USB is probably a good alternative.
When keeping a computer in the listening room, I think the most important aspect is silence. So I would follow Tmesselt's recommendation, or do some research at silentpcreview.com before building a PC.
I am kind of intrigued by the MAC mini because of its silence, but I have never tried it. Regarding PC components, Seasonic power supplies are excellent from a silence perspective, as are Antek cases. You will definitely want a motherboard that supports adjustable speed fan headers if you are not going to go fanless. It is worth it to find a case that supports 120mm fans because you can spin the larger fans at slower speeds and they will be much more quiet than a smaller fan spinning at a higher speed. Same thing goes for the CPU cooler. A large CPU cooler that can fit a large fan will be much more quiet than a stock CPU cooler.
ALan