Good rule of thumb:
Match the native resolution of the display, if it's a digital device.
If it's a CRT...then it depends on the dot pitch. If it CRT, you have to be getting more than minimal overscan from 7-inch tubes — they max out at about 508 lines of viewable resolution (my guess is that you're at double the screen's resolution, and missing every other line if you using a CRT).
When it comes to CRT, often, less is more.