I'm now running Windows 7 Professional 64-bit edition. It's a nice upgrade...very responsive, stable, and it looks very polished.
XNA has been working perfectly so far. I haven't hit any issues, so there's really not much more I can say. I would recommend it.
Oh, and if you have a college or university email address, go to www.win741.com and get a copy of 7 for $30. :)