That is one strange add_image_size() call. You are missing size’s name and mixing up string with integer. And you specify crop mode, when you actually need resize mode. Try this:
add_image_size( 'portfolio-shot', 640, 9999 );
Then fetch it with:
the_post_thumbnail( 'portfolio-shot' );
This should properly register image size under portfolio-shot name in soft-resize mode to width 640px. Note that (unless there were changes in recent WP versions, not sure) new size will be effective only for new uploads.
See this tutorial for excellent writeup on using related functions: