No, never base it on that. The way HDTVs are set up at stores is that thier contrast and a bunch of of other settings are turned way up. Remember that stores have a ton of lights in there and are quite bright. The TVs are not calibrated properly and then when you take the TV home it doesn't look the same. The picture could be all washed out images could look pale, all kinds of things.
Every store I went to the TVs were not optimised and they were usually set to programs that were poor choices to show off HD with. The one cheat that is common is darkening the over head to hide glare and poor contrast. Look at the surrounding lighting.
Walmart, SAMs, had normal lighting. Frye's, best buy and circuit city set up special areas with controlled lighting to make the TV's look better than they will at home so keep that in mind.
The most important thing to look for is how pixellated the image becomes during action or when viewing a non HD format show. This is an indicator of how good the digital signal processor is. I have never seen ratings on the DSP chisp used and they vary widely.
The other DSP item that is really annoying is check into how many options you have to adjust the picture format (4:3, 16:9, 14:9, Zoom, etc.). My set is missing a couple options that are needed and has extra I never use. Most stations (over hte air, sattellite and cable will go through 3-5 formats during one show between intor's, commercials, and the main event. Having the picture re-size automatically is really distracting and if the format control is not good then half the time it will end up badly distorted.
For the price you are pating the store should be willing to get the remote out and let you try the various options and see what the menu offers. Most likely they will look at you like you are nuts if you ask.
Contrast, brightness, etc are usually rated but there are multiple rating systems used so viewing is the best policy.
As for plasma and DLP, I side by side compared them with LCD and was not impressed. I did not see any major improvement in contrast ratio in the black areas. The images were not as bright and when I stood close to the plasma it felt like I was standing in front of a space heater. The DLP has issues with weird patterns forming in the picture. DLP (bulb life) and Plasma (gas leakage) are half the life or less of LCD and weight 2-3 times as much.
PS One of the major plasma panel makers has discontinued plasma panels because they were loosing their ass on them. Plasma has been getting sold below cost because it is dying. That is why they are now cheaper than LCD.