Back in July, I created a simple grid of when to use which Microsoft presentation technology. I did this in response to a workshop I ran at one of my customer's offices. David Chappell just wrote a whitepaper, Introducing WPF explaining WPF and as part of it he also explains when to use which presentation technology, but in much more detail.
Having taken the journey from MFC to WinForms to Silverlight to WPF, I really understand how confusing it can be, especially as you are planning products over the long term. If you care about building great user experiences, please download and read the whitepaper so you can understand the continuum of tools and technologies to support you.
2009-01-29 - Updated link to Introducing WPF.