An iScan Plus (or any of the models that supercede it) will do a very nice job of converting NTSC SDTV to 480p even if the mandatory upscaling to 1080i or 720p is done by someone else. Actually I would prefer one that had component video 480i input as well, I think you need to get the iScan version 2 at least.
If your cable or Directtv subscription includes some analog channels, these channels may or may not look better but will probably not look worse coming to the TV or iScan as composite.
The question you have to figure out for yourself, with the aid of friends, strangers, stores, or magazine articles, is whether your TV's built in scaler does as well or more.
I'm not sure about the Ultra AV you mentioned. Since it is always desirable to audition it first (unless you have friends, strangers, stores, or magazine articles to go by), I would at least suggest you not get it via eBay.
>>> Turdvision, Westa, TDavis
I heartily disagree. There is nothing wrong with a sleek panel TV above the fireplace even if it is playing SDTV. I would hold off on a hi-def DVD player until after I have a HDTV with HDMI/HDCP input (no hurry). No sense getting stuck with a player that loses half the resolution going to your specific TV every time you put in a copy protected DVD.