HTPC Newbie Ethernet Help

[OldWhitey]

I am new to HTPC and have the following situation that I would like some help in resolving. I have Comcast cable on a 3-way splitter at Comcast's box. One cable goes to a modem, a router, and wifi upstairs; cable in a downstairs bedroom, and cable in the HTPC room. We don't need the cable for TV in the HTPC room as we will get our TV from an antenna. However, I would like Ethernet directly to the PC and TV. Running an Ethernet cable to the HTPC room from upstairs is way beyond practical. So, my question is this: Would I be better off with a powerline Ethernet extender, or adding an additional modem and router down stairs, and then running an Ethernet cable to my PC and to my TV? Or, is it really possible to have an additional modem and router on the same cable system? I called Comcast, but all of their customer service is from other countries, as a result of the communication gap I don't really have any faith in what was said or resolved. My loose understanding is that an additional modem can be added to Comcast cable by providing a MAC address to the Comcast Activation group, although I have not determined if this is actually true. Has anyone added a second modem/router to their cable? Or, is powerline Ethernet extender a better idea? Or, is there another way to peel off Ethernet from the cable, or another solution I have not considered?
Thanks for your help.

 

[Cerb]

Or, is it really possible to have an additional modem and router on the same cable system?

Not without paying for another subscription.

Since a HTPC won't need much bandwidth, try a power adapter, and see how it goes. If latency stays below 5ms, and bandwidth above 50Mbps, you'll probably be OK (and if your house's wiring is good, you should be able to beat both of those metrics by quite a bit). They're never recommended, because, like wireless, their performance and reliability are highly variable, but if you want to avoid running a cable, and it's a single point to point install, give it a shot.