If you're one of those guys that are planning to upgrade your systems to Windows 7 or 2008 R2, Microsoft has released two documents that may help you about that task. In my opinion you should (when possible) perform migration instead of in-place server upgrades, but we don’t leave in a perfect world and sometimes things can’t be done the way we want. If you've no choice then these docs will help you to do things in the right way.
Windows 7 Upgrade Paths
Windows Server 2008 R2 Upgrade Paths