Change to differnt Stylesheet dependent on resolution

I have been using style.css to modify my theme and need to know where the link to this is in which file and where. I want to use something like the following to change the templates style sheet depending on the users resolution:

<?php
if ((screen.width == 1024) && (screen.height == 768))
{document.write('<link rel="stylesheet" type="text/css" href="test.css />');
}
else if ((screen.width == 1280) && (screen.height == 1024))
{document.write('<link rel="stylesheet" type="text/css" href="Homepage.css />');
}?>

where would I put this code and would it work ?