conditional homepage

Alright, my website has a welcome page with some copy text on it. When users login this welcome page (the current homepage) must be hidden an become inaccessibel and replaced by the activity stream (the new homepage for loggin in users). Even if logged in users type in the URL they must be redirected to the activity stream.
How can I achieve this? Is there a plugin for this or should i hardcode it?
you can view my webpage at: http://www.massageuitwisseling.net