Err, even cheap 14" portables display 720x576, with the center 640x512 (PAL) or 640x480 (NTSC) fully within the visible area of the screen.
Outputting 320x240 (or thereabouts) will result in a very visible loss of vertical resolution and you may also notice the loss of horizontal resolution on high-contrast images. Video-CD is this quality.
720x576 is widely accepted as the minimum resolution for PAL broadcast quality (720x480 for NTSC, I believe). DVD is usually this quality.
Outputting 320x240 (or thereabouts) will result in a very visible loss of vertical resolution and you may also notice the loss of horizontal resolution on high-contrast images. Video-CD is this quality.
720x576 is widely accepted as the minimum resolution for PAL broadcast quality (720x480 for NTSC, I believe). DVD is usually this quality.