Windows 7 Beta was just released to the public and Microsoft indicates that you need Vista SP1 in order to upgrade.
Even though it says you need Vista I have found a way to upgrade my Windows XP install so that it has most of the new features in Windows 7 by following these steps.
1. Install Windows XP SP3 so that your computer is more secure and running the latest
2. Disable your themes and search services (start -> run -> services.msc) as well as any other services you do not require. After this your computer should be running close to what it will with Windows 7. Also, for good measure, remove any extra non-sense from your startup items. Better yet, do start -> run -> msconfig and uncheck "Load startup items" then click apply.
3. To get that shiny new finder you need to install Directory Opus.

4. For the nice way to rearrange your taskbar use Taskbar Control.

5. You will want to also install Windows PowerShell 2 CTP3 if you plan on using it.
6. You can also install Internet Explorer 8
7. Download Windows Live if you use IM or blog
I think that pretty much covers all of the new features, and I bet the above steps will take you less time to follow than to upgrade to Vista, then to Windows 7. Once it is out of Beta and I have more time I do plan to install it. Until then, the above will get my by.