My son is planning on ordering a 52" Samsung LCD this week.
He has been going back and forth between wanting the 650 or 750.
At Amazon there is less than a $50 difference between the 2, so cost is obviously not an issue.
He would like the 750 because of the added sub woofer. However, he has heard complaints of a halo problem with the 750 (because the bezel goes all the around the set and light leaks through the top). This is not present on the 650.
Apparently, this is noticeable only if the room is totally dark, but this is how he watches his movies and plays video games, so it is a major concern to him.
Some people have fixed the problem by using electrical tape or adding a light source. He doesn't want to do either of these, so right now he is favoring the 650, unless he hears of some type of fix from Samsung in the works.
also
Differences between the LNxxA650 and LNxxA750:
750 has an integrated woofer
16-bit processor vs. 12-bit in 650 (10-bit panel in both)
750 has DLNA features (can stream audio/video/etc. over a home network)
750 can play video off of DLNA/USB flash drive. 650 limited to images/audio
750 comes with 1GB internal flash memory pre-loaded with HD pictures, games, recipes, and a few other things
750's controls on the bezel have a red backlight. 650's controls have no backlight
750 has a square bezel compared to the 650's rounded bezel
750 has a square stand compared to the 650's oval stand
750 stand does not swivel (contrary to Samsung's literature)