Not tested. Probably doesn't even run. Hacked together jquery plugin. Now plus ninja tip

gistfile1.js

(function($) {

  $.fn.credit_card = function(opts) {
    this.opts = opts
    this.change(function(){
      var val = $(this).val()
      if (val[0] == "4"){
        $(opts.visa).addClass("highlighted");
        return false;
      }
      var twodigit = val[0] + val[1];
      if (twodigit == "34" || twodigit == "37") {
        $(opts.amex).addClass("highlighted");
        return false;
      } else if (twodigit == "51") {
        $(opts.mastercard).addClass("highlighted");
        return false;
      }
    })
  }

})(jQuery);