The buy now button is an image form element that submits the buy now form. You could replace:
<input type="image" name="submit" src="http://images.paypal.com/images/x-click-butcc.gif" border="0" alt="Make payments with PayPal, it's fast, free, and secure!" />
with:
<input type="submit" name="submit" value="Click here to purchase" />
To create a regular submit button with your text.