Size: If you read the guides, it says the optimum distance to sit from a TV is 5-7x its diagonal. I've found that you can do a lot closer for an HDTV due to better resolution, but than means that a 48in TV will ask you to sit 12ft away at 3x. Just a thought. These really are big TV's
Technology: LCD has gotten so good it is unreal, so I think plasma is out for both power usage and cost. If you want to spend more, you could go for the new LED tv's with astounding deep blacks, but I find the LCD's are fine and they fit your price range. Definitely get 120hz refresh rate, even if it costs more.
Sound: If you are counting on TV speakers, you are going to miss out on spectacular sound regardless of what TV you buy. You will eventually need a decent five speaker setup for great sound.
Green: Sharp and Samsung use arsenic free glass that can go into landfills. You aren't hearing about this a lot, but in ten years when people start trashing these TV's you will hear more. Other manufacturers may use arsenic free also.
TV reception: Analog looks terrible on TV's this size. Most TV's that you are looking at have QAM receivers that can pick up the free digital signals that cable broadcasters are required to carry for TV stations that use them. You can also get great over the air signals in digital with a set of rabbit ears or a better aerial if you have one.
Movies: Your PS3 will upconvert your DVD's for you, so no worries. You will need an HDMI cable for your PS3 in order to get full 1080p.