There are a few differences.
The A850 is DNLA Certified and will be forward compatible with several current, and likely more applications in the future. It also has the Content Library.
There are a few other very minor differences, but those are the main ones.
--HDTech
I am having a hard time deciding between these two models. I know 850 is slimmer, it has better content and the picture is slightly better than the 650. I have seen these 2 models next to each other. The picture quality between these 2 models is very subtle. You have to look really closely to see that the 850 has a slightly better picture quality. Both TV's have fantastic quality.
However the price difference between these 2 models is quite substantial. Is there something else that I am missing in comparing these 2 models???? The 750 model isn't an option for me to buy.
Any advice is much appreciated.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic