640 is a little bit low for chinese applicants, but might still be competitive at top 50-100 schools as long as you can write good essays and get strong recommendations. It the whole package that matters, so if you are strong at other areas, you might be able to offset a low Gmat. hope this helps
Yes, I think your Gmat score will not add to your application! But if you want to get admitted, you will have to be outstanding in other areas. And you should try to avoid those b-schools that pay great attention to Gmat.