That's a question nobody can answer for you. In my opinion, you can tell a clear difference on ANY high definition set (720p, 1080i, or 1080p) from 37" up. But how MUCH of a difference, how much it means to YOU, and how much does it cost - those are the questions.
A lot of people waited for the "format war" to be settled before jumping in, and this simply isn't the ideal time to buy. Sony has nothing pushing them to lower prices, and HD-DVD is a short-term investment (since there won't be new players or movies coming out).
The answer to your other question, though, is that the stores absolutely don't look as good as yours would with the appropriate cables and very basic calibration. Most of the big box stores are really doing a disservice by sticking up Blu-Ray displays with the wrong cables, wrong settings, etc., and give the customer the impression that this is as good as it gets. In reality, a few minutes of basic calibration and the right cable connections will look much, much better.