if (screen.width<1024) //La considero de 800x600
  document.write("<LINK REL='stylesheet' HREF='css/estilo_planti1.css' TYPE='text/css'>"); 
else 
    document.write("<LINK REL='stylesheet' HREF='css/estilo_planti1_1024_NUEVO.css' TYPE='text/css'>");
/*
  if (screen.width<1280)
    document.write("<LINK REL='stylesheet' HREF='css/estilo_planti1_1024.css' TYPE='text/css'>"); 
  else
    document.write("<LINK REL='stylesheet' HREF='css/estilo_planti1_1280.css' TYPE='text/css'>"); 
*/