There's 2 possibilities for "stretch" here...
1) If you mean stretching to fill your HDTV screen, that's more likely a TV (menu) setting called "stretch" or "fill" or similar. The same would be happening when watching any old non-HD channels, such as those that show classic television or old sporting events not shot in HD. If those too are filling the screen, it's likely you have a TV setting set to do that. Check your settings.
2) If you mean that the visible ratio remains the same (square instead of rectangular) but you just don't like the "blur" when 480-line video is dynamically (more than) doubled to fill a 1080-line HDTV screen top to bottom, you could re-render the video inside of a 1920X1080p "black bars" box. An easy way to do this would be to make a single slide in Keynote that is 1920X1080 and then center the 640X480 video in the center. Set the video to play and save that out as a Quicktime video.
The result is going to be the "black bars" effect on all sides (a black bars doughnut if you will), filling the spaces with blank black bars where you have less video than width or height of screen. On your TV, it is going to look like picture in picture... or like you are watching a little TV screen within a bigger TV screen. BUT, you will be seeing the 640X480 video at it's full resolution (without the TV inventing pixels to try to scale it up to fill the 1080p lines it actually has).
If you are like most people, you probably won't like the doughnut approach, which then points toward the concept of just learning to live with the concept that lower resolution video is going to show on higher resolution screens... just as it does with TV classics and/or classic sports broadcasts.
One last thing: if those home movies were shot on film, there is one other option. Even old home movie film has higher resolution than (perhaps) it's past conversions to DVD or VHS tape. There are services that will scan the film at very high resolution and basically make old films look like HD quality. This is how older movies & TV shows shot on film (instead of tape) can be remastered into HD-quality video. So if you have reels of film (not video tape) home movies, you could use a service to maximize the quality of those so they will look better at greater than 480 lines.