With all of the new product launches forthcoming...things are starting to get muddy...and I found this excellent post from Rob Caron that at least makes a good start for understanding the tiers and relationships between the Express, Visual Studio and Visual Studio Team System products. These are a must read:

http://blogs.msdn.com/robcaron/archive/2005/04/15/408426.aspx

http://blogs.msdn.com/robcaron/archive/2005/04/20/410207.aspx

But, be prepared that there are still many unanswered questions regarding pricing for VSTS and how it fits with existing partner programs and MSDN subscriptions. Microsoft is still working on this story, based on feedback from the business community, so you should stay tuned for more information from Microsoft in the June/July timeframe to provide clarity.