if(nn4)document.layers[dst].src=url;
  if(ie4)frames[buf].document.location=url;
  ..
  if(ie4)document.all[dst].innerHTML=frames[buf].document.body.innerHTML

I prototyped in Netscape Navigator.. had to fix it for IE. Couldn't it be <a href="part.htm" target="dst"> just from the beginning ? - but what you got is AJAX, JS addictions and no caching.