How to add Buy Now At Product Page in Magento - 1 version ?

If you want both Add to Cart and Buy now, add the buy now button to addtoCart.phtml

Go to
/app/design/frontend/[package name]/[themename]/template/ajaxcart/catalog/product/view/addtoCart.phtml

<button type="button" 

        title="<?php echo $buttonTitle ?>"

        class="button btn-cart" 

        onclick="productBuyNowForm.submit(this)">

    <span><span>Buynow</span></span>

</button>

On that buttons onlick event, a custom VarienForm is used, productBuyNowForm.submit(this), which will submit this form and redirect to checkout onepage.

To define this custom form, add the following JavaScript code at view.phtml

Go to
/app/design/frontend/[package name]/[theme name]/template/catalog/product/view.phtml

<script>

var productBuyNowForm = new VarienForm('product_addtocart_form');

productBuyNowForm.submit = function (button, url) {

    if (this.validator.validate()) {

        var form = this.form;

        var oldUrl = form.action;

        alert(form.action);

        if (url) {

            form.action = url;

        }

        /* add return Url */

        var inputreturn= document.createElement("input");

        inputreturn.type = "hidden";

        inputreturn.name = "return_url";

        inputreturn.value = "<?php echo Mage::getUrl('checkout/onepage')?>";

        document.getElementById('product_addtocart_form').appendChild(inputreturn);

        /* add return Url */

        // Append a line break 

        var e = null;

        try {

            this.form.submit();

        } catch (e) {

        }

        this.form.action = oldUrl;

        if (e) {

            throw e;

        }

        if (button && button != 'undefined') {

            button.disabled = true;

        }

    }

}.bind(productBuyNowForm);

</script>