How big do you want? Don't get too caught up in all the LCD/Plasma comparisons - these days both are very similar in picture quality and you won't notice the difference. However, there is a difference in price. If you are looking 50" or above then Plasma is significantly less expensive than LCD. Below 50" and you'll probably end up with an LCD. Of course, this is all assuming you go with a flat panel.
You'll have trouble finding a large flat panel in your big box stores that is 720P since they are all moving to 1080P. That being said, other posters are right that none of the over the air broadcasts, cable, or satellite providers broadcast in 1080P. It is only useful if you add a blu-ray player or are a gamer. And in those cases, you still won't really be able to tell the difference between 1080P and 720P unless you start looking at projectors.
Some other advice. DO NOT BUY ANY CABLES FROM THE BIG BOX STORE!!!!!!!!!!!!!!!!!!!!!! They are way overpriced and you can get them for 10% of the cost at
www.monoprice.com
Also, when you set the TV up at home, put it in "cinema" or "movie" mode. The default mode on these things is "vivid" where the colors are far too bright and no where near properly calibrated. All the TV's in the store are on vivid mode because buyers are impressed with bright, shiny objects.
Also, you will probably want a programmable remote control. Because the new TV's use lots of different inputs, you'll want something to make it easier for the family to switch between movies, games, and television. I highly recommend harmony remotes.
Lastly, buy the TV now. Before she changes her mind.
-Mike