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In VS 2005 at least, on the Team menu you will find a Project Alerts... item which allows users to specify an email address that will be notified when My work items are changed by others, which covers both the situations you mention. I imagine VS 2008 will have a similar thing.
I know your post is for 2008, but it's an old post and hopefully you're on 2010 now. For TFS 2010 there is an easy solution for you now, via a plugin which can be downloaded from CodePlex - Team Alert
It's a simple copy-paste solution which can take you 5 minutes to put in place using the configuration extract listed in the post below:
This post will show the exact configuration you need to perform what you want. Notify AssignedTo user of new work (for a specific TFS project)