IE6 is turning into a C grade browser this quarter (http://yuiblog.com/blog/2010/11/03/gbs-update-2010q4/) so you can probably just not serve JavaScript to it (server side detection). IE7 still doesn't do parallel script downloads, so you might need to do bottom loading for it, but it's best to first find out what your users use before optimizing for it. You can get really complicated and change your page structure based on useragent and user's network characteristics, but that requires far more development effort.
