For the best sound get a good proaudio card like by RME, Lynx or Digital Audio Labs. Use the digital output to connect to an external DAC. My card is the DAL CardDeluxe and it supports toslink, RCA coax, and AES/EBU via a TRS adapter. Other cards support USB digital output which I hear is very good, but I have no experience with it.
Don't think that you are just going to add a sound card to a PC and get great PC Audio. You need to be concerned with power supplies, power conditioning, digital cables, external DAC, and the actual OS configuration tweaking of the PC.
Don't think that you are just going to add a sound card to a PC and get great PC Audio. You need to be concerned with power supplies, power conditioning, digital cables, external DAC, and the actual OS configuration tweaking of the PC.