How to stop / hijack a infinite redirection loop with a redirecting rule
You might want to consider a simple redirect in .htaccess

To redirect based on user agent, if not already on redirected page.

RewriteEngine On
RewriteCond %{HTTP_USER_AGENT} MSIE [1-7]
RewriteCond %{REQUEST_URI} !^/upgrade/
RewriteRule .? upgrade [L]