Ethernet connection for best sound quality

"you don’t need a dedicated streamer (vs a PC) for top notch performance".

I’m going to get technical here, but there are many cases where a PC’s USB or HDMI output is very high in noise due to a ground loop which they are prone to due to the switching power supply not really designed to play nice with others. In those cases there can be audible hum/noise or significant jitter. Multiple sources including Stereophile and Audio Science Review have shown this. I’ve also experienced audible hum when using a PC source and USB.

There are multiple known cases where a laptop source measured significantly better when unplugged from the wall.

So.... personally I would not use a PC or laptop as a source to avoid these issues altogether. I use a Raspberry Pi with a high current iFi wall power adapter.

You can get around this issue a little by using a USB isolator (sucks not all DACs are completely isolated) or an optical S/PDIF output but you still have the issue of the amounts of power supply noise making it into the AC.

So generally, I agree that a PC is never going to be my first choice as a streamer, but you could maybe make it OK?

Don’t need a dedicated Ethernet card really. Your Ethernet cable should match your router’s port speeds.  If you use cat 5 on 1 Gig ports you can have degraded and inconsistent performance.  Having said that, 100 MBPS is way more than any streaming service needs, but most routers have at least 1 Gig ports. If your router has 10 Gig ports you must use cat 6a or better.  

Shielded is preferred IMHO to minimize chance of radiating to your interconnects.

So from wall to modem to router to streamer to dac, correct?

Often you have a switch near your streamer to accommodate your TV, Roku, etc. as well, so:

Modem --> router --> switch --> streamer --> DAC

If your modem has a coaxial input consider an exterior gas discharge surge protector, and an Ethernet isolator near the streamer. I’m particularly obsessive about surge protection being in a lightning prone area so I use both. Ethernet isolators are particularly good at the end of 30’ or longer Ethernet runs to reduce induced surges breaking through to downstream gear.