I have not made the move to Windows Vista, so I sure do appreciate Windows XP SP3 to avoid the 80 to 90 Security Updates one needs to install after installing Windows XP SP2.
Windows XP Service Pack 3 (SP3) includes all previously released updates for the operating system. This update also includes a small number of new functionalities, which do not significantly change customers' experience with the operating system. This white paper summarizes what is new in Windows XP SP3.
You can check out the white paper with a summary of the changes in Windows XP SP3 here.
You can learn more about how to grab Windows XP SP3 if you are an MSDN Subscriber here.
Hope this helps.
Dave