Most TVs when you first get them are set at a vivid configuration so they will grab your eyes but they aren't very realistic and very good for long time viewing.
Here are a couple of reviews with their settings which should get you into the ballpark. I'd read the reviews and get an understanding of why they set what they do and why they turn off some functions.
http://ultimateavmag.com/flatpaneldi...52/index7.html
http://reviews.cnet.com/flat-panel-t...&tag=mncol;lst
http://reviews.cnet.com/9602-12576_7...sageID=2510694
The 650 and 750 are pretty much the same TVs and the size shouldn't matter.
and here's a thread on the settings and calibration.
http://www.avsforum.com/avs-vb/showt...msung+settings