There already is a 64-bit version of Windows. I have it. It is Windows x64.
Vista comes in a 32 bit or 64 bit beta, and it may ship at retail in both versions as well. However, it doesn't have any real noteable new features, and won't ship til 2006-2007. All the main features they were talking about two years ago like the new file layer, etc. have been cut.
There is a new API for running full Windows apps without installng them remotely via Internet Explorer. Hello, Java basically offered similiar functionality years ago before M$ hosed it, but now M$ is making a proprietary version. But didn't they already do that with ActiveX? They don't know this will be done in time for launch, and mark my word, it will be a bigger security nightmare than ActiveX.
The new file layer to help searching you can download a beta of for Windows XP and doesn't require Vista. And Google Deskstop Search replaces this with freeware.
There is also DirectX 10, but you'll be able to download that for XP.
The main new feature is a better looking GUI called Aero. Too bad Aero won't work as well or as fast on NVidia cards. The main new feature of Aero is window transperancy. Yet, I have that in XP with Style XP.
Vista looks to be a real worthless upgrade that will just eat system resources and run slower.