There are a few important things to consider before buying a HDTV, find out the basics you need to know to make the best decision and get the most for your money.
When you got to your local electronics store, don't be fooled by the picture quality of all the different TVs. Don't get me wrong, yes some are better than others but, most if not all of the TVs settings are modified for the low light showroom environment. What may look like a good picture there, may not look quite as good when you get it home.
Another aspect to keep in mind is the display size. The display screen size for a Wide Screen HDTV is smaller than the screen size of your CRT TV, if you still own one of those. A good rule of thumb is to buy one size larger than your current screen size (CRT = LCD/Plasma) for it to look the same.
HD looks best on a large TV, so buy the biggest set you can afford, but make sure it fits the space. A 37-inch screen may work best in a bedroom, but a 60-inch screen is ideal in a large entertainment center.
The new rave is hanging the TV's on the wall. I personally haven't done it yet, as I would need to buy an additional piece of furniture to house all of my audio & video equipment (DVD Player, DISH Receiver, Surround Sound, etc.) But, it sure does look good!
Make sure the TV you buy is a "Widescreen" TV, as all HD programming is in the widescreen aspect ratio (16X9). This really isn't a problem because most HDTVs are widescreen compatible but, just had to tell you.
You will need a HD DVR or HD TiVo to record HD programming and play it back in HD (on a HDTV of course). Standard definition programming on a HDTV looks pretty bad. At first you say its not that bad (trust me I said it) but, eventually you come to grips with reality, its bad.
If I could recommend anything, buy a HDTV that is capable of displaying a 1080p signal. They are not that much more than a 720p HDTV (I regret buying my 720p HDTV).
As far as warranties are concerned, I usually don't buy them as every time I do, its been a disaster trying to get them to fix it or they want an outrageous amount to ship it back to them and if I never need to use them, it just makes me mad I spent the extra money.
Take some notes of what you currently have (audio & video devices), and how you want to connect them (HDMI cable, Component cable, etc.) and make sure your new TV has all the inputs you need.
With a little research, you can find the best HDTV for your home that will meet all your needs!