Well we bought the 52 inch 120hz 1080p 650 LCD. Not going to take delivery of it until we move into our house.
Main differences from what my short research was 850 was the thin one and like the 750 has built in hard drive that has games and pictures. That was it.. The all had 120hz they all were 50,000:1 contrast ratio. The housings were the only real difference. Picture quality in all three were all the same from what I could tell. They had the 650 and 850 side by side and there wasn't anything different that I could tell.
Here it is:
http://www.sears.com/shc/s/p_10153_1...ame=Flat+Panel
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