When you start talking separate DACs, you start getting into some very nit-picky stuff: stuff that some people swear they can hear even though it can't be measured, or stuff that can ONLY be measured, but isn't actually audible.
If you go the route of using a home theater receiver in your setup so that you can easily connect and switch between all of your sources (PC, Cable Box, game consoles, etc.), then personally, I would simply use the receiver and not worry about separating out the components to the level of separate DACs and such.
If you go with the Onkyo TX-NR709 that I suggested in your other thread, you'd be getting very capable Burr-Brown DACs in that receiver. I'm not saying they're the be-all, end-all. But personally, unless you are mixing and editing music at a professional level, I honestly don't think you'd find any additional value in an external DAC. For the purposes of simply listening to playback of music, TV and movies, the DACs in the Onkyo receiver will do a more than adequate job.
Again, I'm not certain what output options you have available from your PC. If you have HDMI output and the proper software, you should be able to output up to 7.1 192kHz/24-bit uncompressed PCM. If you are using a S/PDIF or optical digital audio output, you can use up to 2-channel stereo 192/24 PCM. And if you are using 5.1 or 7.1 analogue audio output, then you are relying on your PC's soundcard to act as the DAC.
Now that last option might be why you are considering an external DAC. If that is the case, I would actually recommend that instead of an external DAC, you consider upgrading your sound card or video card so that you can have digital audio output from your PC. If you upgrade your video card to one that offers HDMI output, then you can get your digital audio to the Onkyo receiver that way. If you upgrade your soundcard, you could use digital S/PDIF or optical.
Bottom line is that I would allow the home theater receiver to handle the digital-to-analogue conversion. If all you have right now are analogue audio outputs from your PC, then you'd wind up having to use a USB external DAC or upgrade your soundcard to one that offers digital output anyway. So my advice is to simply make sure that you are using a digital audio output from your PC and then let your new home theater receiver handle the DAC duties.
Hope that helps!