Re: big box store not knowing about QAM
The sales associates are untrained and are selling hype with little knowledge of what technology would benefit the customer the most. Shop elsewhere where there are informed and trained sales staff. I would not be shocked if the same store sells things at near full list price, and/or carry and pushing some outdated early generation models to less informed buyers, so they are either uninformed or trying to move yuor attention away from the technical specifications that matter (but they may not sell in their most profitable models or in dated inventory they are trying to clear out).
QAM (cable ready HD) is one of the more important things about selecting an HDTV. Read about it at: http://en.wikipedia.org/wiki/QAM_tuner where it describes that "An integrated QAM tuner allows the free reception of unscrambled digital programming sent "in the clear" by cable providers"
The older HDTVs don't have it, and these should be avoided, since you want a future proof investment and that means getting a QAM receiver, getting real (full) HD 1080p, and beyond that, just go for the best value you can find (high rated picture quality at excellent pricepoint and sufficient HDMI inputs 2=OK, 3=better).