You're not alone - probably the biggest wake-up call for new rptv owners is when they get it home, hook up the cable and say to themselves "It looked SO much better at the store" without realizing the stores usually feed all their sets a hi-def signal.
IMO, "digital" cable is a worse option than plain old standard cable. In most cases, it's the same source material that is digitally compressed (enabling the addition of extra channels/features) and then decompressed in the digital cable box. In my experience with digital cable, the compression artifacts, haloing, banding, etc. was deplorable. Even on a 27" direct-view set, I could see artifacts, so it's a given that it will look even worse on a 57" set.
Picture quality will probably be improved by switching to satellite (I can't offer any firsthand experience, since my apartment faces the wrong way and I've been forced back to cable

).
As for the banding line rolling up the screen, that sounds like a ground loop problem. Others can probably offer specific advice on how to troubleshoot that, but I'd try disconnecting other gear and just hooking up the TV and cable box and see if that helps. You can also probably search the forum for "ground loop" and find some good advice.
I am a huge advocate for satellite TV, and it pains me to no end that I have to use cable once again. I think a lot of cable companies say they'll have HDTV support "soon" but my gut instinct is "soon" probably isn't soon enough for most people. Screw the cable company - get satellite!
-Jason