I've finished my little proof of concept application that's using Windows Presentation Foundation. And I'm EXTREMELY pleased with the results, performance under Windows XP feels just the same and I guess it should be better once I actually optimize the handling of media.
All in all, while I was once afraid and skeptical about using WPF as a platform for writing Windows Client applications making the POC has changed that mindset for me. And now I'm glad I have WPF as a tool in my belt.
Though I wish they'd make it easier to hide the darn text input caret for the RichTextBox! Had to go through an ugly ugly hack just to do that!