You can always have a still image that gives the illusion of being 3D, and there are some hacks to let you use a screensaver as a background, but I would seriously recommend against it. They get real old, real fast. Download a Knoppix CD and run XSnow for awhile to see what I mean.

Also, Microsoft has been busy chucking feature after feature overboard for Longhorn in an effort to try and get the thing shipped by sometime in 2006. I wouldn't go getting all excited about Longhorn having Feature X, because it may well have been chucked overboard to try and meet deadlines. And given all the trouble MS has been having getting Longhorn ready, I think I'm going to probably hold off upgrading... Assuming I don't just buy a Mac Mini and leave my current desktop as-is for gaming purposes.