You may want to look into a firewire to spdif converter so that you aren't limited to dacs which only have usb input. I just picked up an M Audio Firewire Solo, which I have connected to a Musical Fidelity XDAC V3 and it sounds pretty sweet.
I have a Benchmark DAC hooked upto a squeezebox in my main rig and the Benchmark sounds pretty good also.
If you have USB 2.0, a USB DAC may be okay. If your PC has USB 1.1, I'd go with firewire (if your PC doesn't have firewire, you can get a PCI firewire card (go with the texas instrument chip). USB was never designed to transfer high multimedia loads-it was designed to replace parrallel and serial ports. Also, firewire (6 pin) uses PC power and no need for an outboard power source - and doesn't bottleneck like a USB port may.
I have a Benchmark DAC hooked upto a squeezebox in my main rig and the Benchmark sounds pretty good also.
If you have USB 2.0, a USB DAC may be okay. If your PC has USB 1.1, I'd go with firewire (if your PC doesn't have firewire, you can get a PCI firewire card (go with the texas instrument chip). USB was never designed to transfer high multimedia loads-it was designed to replace parrallel and serial ports. Also, firewire (6 pin) uses PC power and no need for an outboard power source - and doesn't bottleneck like a USB port may.