You're getting into an area that can cause controversy--many people claim wire is wire and that in double blind tests people can't tell the difference between uber-high end wire and coat hangers. It is even worse because there are undoubtedly companies out there that make a lot of money selling snake oil claiming vast improvements.

I think I can hear a difference in my system, so I spent some money on wire. Maybe it's placebo effect. Maybe it's real. Either way, I tend to think it was worth it for my system and my ears.

So, what I'd counsel is getting/borrowing some "better" wire. Listen to your system, then run the better wires across your living room and see if you can hear a difference. If you can, and it's worth it, replace the wires in the wall. If not, you won't have lingering doubts.

Even pre-DTV, a lot of TV shows were surround sound over the air. With HD, and even SD, you should find a lot of programming that will use the 5.1 modes. I'm assuming you will hook up your CATV box to your receiver using HDMI or digital optical/coax. Just make sure your set top box is set to output surround sound over the digital output, rather than PCM, and your receiver should do the rest if surround is available.