It's 800x600 at 60Hz don't go to 640x480.. Best way to do all of this is to make sure the:
HDTV has VGA port
VGA port on the laptop
Make sure that the laptop LCD is showing what you want to see on the HDTV VGA. Use the Function key to switch views. So you only go it on the HDTV. Those HDTV that just don't have VGA you could use S-Video at 1024x768 60Hz.
Better to use a HDTV VGA Port as that is designed to work as a computer monitor also.