all the 7" screens from mp3car.com store have a native resolution of 800x480 (16:9 Aspect ratio). The 8" screen is 640x480. They can display higher resolutions by scaling down the image, but it doesn't look as good (important for things like reading street names on a map program)

Personally, I'm going to go with a larger screen with a higher native resolution (12.1" 1024x768) from Datalux. They're about the same price (see them on ebay for under $300) and have a touchscreen, but don't have the other stuff (RCA video inputs, built in speaker, remote) that i won't use anyways. also won't fit in the dash.