I would look at the card in your computer as the starting point and as such if it gets the data stream (mostly) right you'll have fairly good results down stream with an outboard DAC (as long as it's built well and does not have any grievous character traits). The hard part and a lot of controversy surrounds which output standard is good enough or of high enough quality that it will yield the desired result that you expect or want.
IME thus far I have found that SPDIF (COAX) comes closest to a stand alone dedicated system (CDP/DAC combo) with the far and away largest accepted HQ method comes from the Pro recording world , Fire wire from comp and BNC to outboard master clocks and DAC decoders. TOSlink and USB being the lowest quality in terms of listening tests. COAX SPDIF is the sweet spot IMO that can be implemented without breaking the bank so I would aspire to setting up a comp system based on that output first and foremost.
That being said some considerable push back of late regarding some USB based solutions claim equal results with SPDIF/Firewire. Personally speaking I cannot confirm nor deny those results since I've not tested any USB based solutions beyond the ordinary type that is in widespread use nowadays (The PCM2707 is the most common and really is nothing special) and as such a PCM2707 based unit is not going to cut it vs SPDIF.
Consumer audio solutions that are not expensive (compared to Pro audio at least) that will supply a COAX out (a good one at that) include the AuzenTech X-Fi based PCI boards, HT Omega Claro + for starters.
A hardware accelerated solution is far more desirable than a CPU cycle chewing software based or motherboard chipset integrated solution, so don't be fooled by mobo marketing that claim HQ/HD sound support that is the equal of a separate PCI sound card. The marketing and reality rarely meet. There are so many negatives to on board audio in a mobo it's tough to know where to start. The simple solution is to avoid on board altogether.....and put that money into into a decent PCI/PCIE card dedicated to sound.
As always read up on a number of affordable solutions that will satisfy your needs. I'm perfectly happy with the Auzen Prelude PCI card (using it's COAX out) for the casual listening system
Price is around 155-175US shipped depending on where you look. The Claro card is just as good as the Prelude from reports on it's performance as a transport.
All of the above mumbo jumbo is IMHO of course ...
Good luck.
Peete.