I have had some considerable experience with sound from a "PC". As far as computer power supply is concerned, linear power is better. The HDPLEX options are recommended, but they are external units and will require some modifications on the power wiring to your computer components.
If you are looking at normal PC power supplies, I would recommend getting the largest you can afford. They are all switching power supplies, so the larger sizes will have larger capacitors as a power bank. This equates to smoother current/power. The Corsair AX1600i measured to have the cleanest power output. However, the cables that Corsair supplies are heat-shrink and techflex coated. These are extremely thick and stiff cables. If you are trying to route these behind the motherboard in small spaces, the Corsair cables will NOT work beause you just cannot bend/flex the cables enough to fit through tight areas. The Seasonic PRIME TX-1600 is almost as good and they have much better cables for routing/bending/flexing.
For transfer of signal to your outboard DDC or DAC, I have found that USB quality is really dependent on how good the USB-to-i2s circuits are on the DAC. I, personally, don't think the USB is the best mechanism, but this is a controversial subject. I have a friend who owns a $25,000 Pink Faun music server that includes both a Pink Faun USB card and a Pink Faun AES/EBU card (both with the highest level OCXO clocks). When using the USB output, it gave the highest resolution for midrange and high frequencies (air), but it just did not sound as natural and full when compared to the AES/EBU output. This was going to a $16,000 DAC (I can't remember which one it was). The AES/EBU output had more weight, bass strenght, and overall fullness - plus it sounded more natural and real.
On my own system, I am running a stock level version of the Pink Faun AES/EBU card with a Farad Super3 power supply. This is just outstanding. The Pink Faun s/PDIF card with AES/EBU and BNC outputs would be my recommendation if you have that input on your DACs. That being said, if you are running a Linux operating system with Intel CPU, this card does have problems. Running Windows on Intel should be fine (it's a weird chipset comparability issue).