I am getting a very similar problem. Did you finally solve this?
I am running LMCE with an nvidia FX5700 chipset. Display is through the VGA port directly to a Selectron 42 inch LCD TV. When I set the resolution to 720p in the video setup wizard, which should give me the best image for the 1366x768 native resolution of the LCD screen, the resulting image is slightly larger than the screen vertically, but it is noticeably larger horizontally and off center to the left.
It seems like the video card is generating a 1366x768 image, but cropping it to fit the 720p resolution (1280x720). This does not affect the GUI for the LMCE, because the setup wizard asks to adjust the image size for the GUI. However, everything else --Mythtv screens, KDE, Computing -- is shown off center, shifted to the left, and cropped at the top, left and bottom edges.
I have been looking everywhere but don't seem to find how to correct this discrepancy in image sizes.
I don't have the same problem with 1080i resolution, but the system becomes kind of sluggish at this resolution, so I'd prefer not to use it. This problem is not present when running the same hardware under Windows XP at the same resolution.
Can someone shed a light to what I may be missing?