Change URL of button for 480 width Javascript

I'd like to change the url of the button when they hit submit if they are on a mobile screen 480 width or less.

Example:

<form name="myform1" action="mylink.php" method="post"> 
<input type="image" name="submit" src="/Button1.jpg" border="0" alt="Submit" />
</form> 

But on mobile devices 480 wide or less I'd like it to change to:

<form name="myform1" action="mylink-mobile.php" method="post"> 
<input type="image" name="submit" src="/Button1.jpg" border="0" alt="Submit" />
</form> 

The page is PHP but I'm thinking I can use javascript screen.width to detect <= 480 but I'm not sure how to dynamically change the form action url based upon the screen width. How would I do this?