Bob - Streaming from computer to a DAC external to the computer will give you better sound quality than trying to use the computer's DAC and running from the headphone jack into your 2 channel system. The quality of that outboard DAC will definitely influence final sound quality. You can spend thousands on an external DAC...but there are some very good options for under $1000. Many newer integrated amps and even CDPs are being offered that have high quality internal DACs, so that's another wrinkle to consider.
Personally, I listen to on-line music...e.g., Spotify, Tidal, Pandora, YouTube via a MacBook Air that's connected by a Pangea USB cable to a Musical Fidelity V-Link 192 USB/SPDIF converter and from that to a Schiit Gungnir DAC via Stereovox digital coax cable. I connect the Gungnir DAC using Morrow Audio interconnects (MA-3? MA-4? don't remember) to my pre-amp or to an integrated amp.
More and more DACs can accommodate a USB input directly. If such is the case, a USB/SPDIF converter might not be essential or provide any benefit depending on how the DAC is designed. Hope this helps.