validating the textbox multi line

<script src="Scripts/jquery-1.4.1.js" type="text/javascript"></script>
<script type="text/javascript">

    $(document).ready(function () {
        $("#MainContent_txtFirst").keypress(function (e) {
            var strr = document.getElementById("MainContent_txtFirst").value;
            //var strrlen = strr.length;

            if (strr.length > 349 && e.keyCode != 46 && e.keyCode != 38 && e.keyCode != 39 && e.keyCode != 39 && e.keyCode != 37 && e.keyCode != 8) {
                alert("Only 350 characters are allowed");
                return false;
            }
            });
    });

</script>