#global-header {

  .header-wrapper {
    // Prevent header resizing in IE8 and below
    @include ie-lte(8) {
      width: 960px;
    }
  }
}

#global-header {

  .header-wrapper {
    // Prevent header resizing in IE8 and below
    @include ie-lte(8) {
      width: 990px;
    }
  }
}

Test header width in IE6
Test header width in IE6
