Here's a link to a press release on microsoft.com about future and upcoming windows technology innovations. In it they mention "Longhorn", the new version of windows, "Avalon", its graphics subsystem, and a new version of Visual Studio, "Whidbey". I believe Longhorn is to be the next version after XP, with the 64-bit version of XP being released this winter sometime.
The 64bit version of windows server is already availible for IA-64, the 64bit version of windows XP for the Athlon 64 is nearly complete and will be done soon, and availible sometime early next year. The next big windows release is "Longhorn" and that is due out sometime in 2006.