Welcome to the forum.
You might check to make sure that the Hi-Def box is outputting a HD signal. On the remote, there is an INFO button. Press that, and you'll see a box appear with the incoming resolution. What are you seeing when it displays an HD channel?
If it's not 1080 or 720, you might want to call your cable provider and have them walk you through increasing the resolution OUTPUT from the box to the TV. Many boxes come default with 480 (Standard Def) because HD and SD televisions can see a SD channel. But SD televisions cannot always process an HD channel, so the company sets the boxes at a lower resolution by default.
Also, the cable box (like the television) also has a picture size setting. Set your TV at 16x9 on an HD channel, and then go into the menu and adjust the OUTPUT picture aspect ratio. Again, someone from your cable company may have to walk you through this, but they'll know their box menu far better than I would.
Also, what cabling are you using? HDMI, Component, or other? You'll definitely want to use at least component, and HDMI is probably a better option. I'm assuming you're probably already on board with that, but wanted to throw it out there just in case.
Keep me posted.
I just purchased 2 of these bad boys and have one set up (the other is still in the box). It's connected to a Comcast high def box and the picture stinks with 3-4 inches of black bars on top and bottom and what is on the tv is grainy and can't keep up with the motion. something has to be wrong as this tv is too good to look like this. I've yet to see the full picture be utilized and have watched a lot of HD programs on HD channels being broadcast in HD - including all football games this weekend and Monday night (CBS, NBC, Fox, and ESPN). I'm not sure if this is a Comcast problem or a Samsung problem. I've checked the picture size through my Samsung remote and it's on 16:9.