I want to only allow users with IE8 (not IE6, IE7) or another browser to access my site when logged in.
I followed: http://code.google.com/p/ie6-upgrade-warning/ But
Personally I would recommend users to upgrade IE for security reasons. I would use a conditional comment to target browsers below IE8.
This message should be unobtrusive but graceful. You should also make the website usable for those that cannot upgrade their browsers.