Hey Aandagt,
have a look at Featured Video Plus . It does exactly what you request here automatically if your theme uses the_post_thumbnail() or get_the_post_thumbnail() to display featured images. I wrote it after using the original Featured Video plugin.
If you want to use your code, alter
<?php if (has_post_video()) the_post_video(640,360) ?>
<?php if (has_post_thumbnail( $post->ID )) {to
<?php if (has_post_video()) the_post_video(640,360) ?>
<?php elseif (has_post_thumbnail( $post->ID )) {Alex