var drop2 = $("select[name=drop2] option"); // the collection of initial options
$("select[name=drop1]").change(function(){
    var drop1selected = parseInt(this.value); //get drop1 's selected value
    $("select[name=drop2]")
                     .html(drop2) //reset dropdown list
                     .find('option').filter(function(){
                        return parseInt(this.value) < drop1selected; //get all option with value < drop1's selected value
                      }).remove();  // remove
});

http://jsfiddle.net/HTEkw/