I have Comcast "triple play" using an Arris cable router/modem. The Arris is connected by a "plug link" to an electrical socket. Two computers connect to the LAN via plug links. My main computer is separated from the Arris by about 75 feet with a lot of walls in between resulting in a very erratic internet connection. I just purchased a Netgear range extender with the thought that I would connect wirelessly (like my laptops) from the Arris. For wahtever reason, the range extender doesn't seem to extend range. I used my laptop to test the signal and, of course, it connects easily within 30-50 feet of the Arris. However the laptop doesn't appear to be getting any signal from the range extender as the signal degrades immediatly adjacent to the extender. My thoughts now are to purchase a second cable modem and place it close to the main computer and connect it by ethernet. I don't know if this will work with two modems working off the same Comcast cable signal. I am not a "techy" so would appreciate any thoughts.
I have a similar problem to yours... I ran an ethernet cable from my wireless router (which sits right next to my cable modem) to the center of my house where I put in a wireless access point. I use a linksys WAP610.
Many systems will not allow two modems to be used at one address. It's best to call in to see what the local policy is. Even if they do allow it you will need to pay for two accounts (twice what you are paying for HSI now). In addition, the signal levels/line quality has to be adequate enough to allow for the losses that are associated with the necessary additional cable split.
Powerline ethernet adapters may be an option for you. Google for more info.
The powerline adapter may be too "techy" for me. By the way, my plug link is connected by an extension power cord; I'm wondering if that degrades the signal as opposed to plugging the plug link directly into a wall socket. Perhaps I should try a longer ethernet cable that would allow me to directly plug into the wall socket.
I used to install internet/TV for AT&T Uverse, and I have had [i]terrible[/i] experiences with plug links. I would either move your computer closer to the router, or find a non-being-strangled-to-death-by-wife way to hardwire it directly to the router.
If you have a crawlspace or unifinished basement, you can run it through there easily. If not, maybe you have an attic, and can fish it through the walls. Worse case scenario, run it around the outside of the house. Any of that will work.
If you don't feel comfortable running the wire yourself, you can call your local PC store, and they will either run the wire for a fee, or suggest someone who can.