the current trend is to go 'direct' from usb to dac (usb to i2s and i2s to dac, all inside 1 box).
more flexible is to go usb to spdif, then out of the box and into another box, the spdif/dac. those 2 are more standard functions and give as good sound in almost all cases as the 'direct' usb method.
if you do go with usb, the new format is called 'uac2' (usb async audio, class 2). class 2 means 'pull data' model; and that has the dac pulling data out of the computer, as it needs it, with the dac doing the local clocking of the digital data to analog. uac2 can support 192k where uac1 stops at 96k (if you care). uac1 pushes data from computer to dac and the timing is derived from the host cpu and usb data stream, which is not optimal. we're talking a percent or so, in diff, so nothing huge at all. but if you are buying new gear, look for 'driverless' uac2 usb/dac devices.
I've had good enough luck with $10 usb/spdif dongles, though. if you care only about 44/48 audio and not 'high res' the $10 usb cm102 (cmedia) are fine enough. they are the chips inside the gear, so you'd want to see what chip the usb/spdif dongle is using. cmedia is good, reliable, cheap and supported on all platforms.
once you get the usb/spdif side done, you now have the rest of your time and money to spend on dacs
but again, even a $20 dac is probably going to be hum-free and noise-free compared to anything inside your pc.
one exception is the juli@ ('juliet'? strange company or product name) sound card. its pci and now pcie. from what I'm told, its studio quality (ie, even measurement quality). its onboard a/d and d/a are as good as it gets under $1k and it also has spdif out. if you are looking for the best non-pro audio card, that would be the one I'd pick.
but, really, usb is the new way to go. usb2 (don't need 3) and do NOT use the last 2 intel chipsets; sadly, they messed something up and I've had nothing but trouble with both linux and win7 (so its not really the os, but its the haswell chipset and the one before that). after that, I don't know. way before that, things were fine. but some intel usb chips really suck when it comes to audio and realtime data. not sure why. I end up using older mini-itx fanless pc's for my sound system